On the night of Jan. 11 in a crowded, shabby, concrete room at the state penitentiary in Avon Park, Fla. the TV will be turned to Mercy or Murder. The NBC movie is based on the crime of the prison's most notable, and perhaps unlikeliest, occupant, Roswell Gilbert. In 1985 the former electrical engineer killed his 73-year-old wife to end her long suffering from Alzheimer's disease and osteoporosis, a degenerative bone disease. While Gilbert plans to watch the TV drama, he usually prefers to keep to himself—reading novels and grappling with the severe depression that has plagued him since he began serving his jail term in May 1985. "The cloud hangs over your head—'life sentence,' " says Gilbert, who at age 77 can hardly expect to outlive his 25-year sentence for first degree murder. Turned down in a plea for clemency in August 1985, he won't be eligible for release until 2010.

It was on March 4, 1985 that Gilbert loaded a 9-mm Luger, part of his extensive gun collection, entered the living room of his Fort Lauderdale condo and quietly walked up behind his wife, Emily. He shot her in the temple, then, after feeling her pulse, reloaded the gun and fired again. It was the second shot that convinced a jury that Gilbert had deliberately killed his wife—to rid himself of a nuisance, said the prosecutor—rather than in the uncontrollable despair that might have brought a lesser conviction of manslaughter. Florida aw leaves no room for the third possibility: that Gilbert deliberately chose murder as a moral rather than a malicious act. "I just could not allow my lovely lady of 51 years to descend into a hell of suffering and degradation," he wrote in a note he pinned to the bulletin board before he summoned the police. But Gilbert's chilly demeanor in court—where he answered the questions put to him in a dry, factual tone—hardly conveyed a sense of love or compassion. "The jury felt his manner in court made him seem cold and insensitive," said Martha Gilbert Moran, now 52, Emily and Roswell's only child. "But that's just Daddy. He was of a generation where you didn't express emotion."

Everyone who knew the couple, including their daughter, insists that Roswell was a devoted husband. During their first decades together he happily spent the bulk of his comfortable income to indulge Emily's taste for the good life. Later, as she steadily deteriorated, he went to extraordinary lengths to maintain a semblance of normal life for her. "Daddy used to put on her makeup," said Moran. "He would not allow a lot of these things to be brought out at the trial because they were too private."

Gilbert has also kept his distance from Mercy or Murder. Though writer-director Steve Gethers talked with him for several hours in prison, the film script was created largely from court transcripts and press accounts. Gilbert, who was paid $50,000 for the TV rights to his story, had no other official relationship to the project. But as its air date approached, he agreed to talk with correspondent Linda Marx. Charming and unusually candid, Gilbert first revealed the agonizing details of his life with Emily and his decision to end her days one quiet Monday afternoon.

Emily and I were madly in love for 51 years. We had a nice life, a big house in Montclair, N.J. Emily didn't like to cook and I didn't care. We ate in restaurants and had a housekeeper who helped raise Skipper [daughter Martha's nickname] before she went to Vassar. We belonged to country clubs, traveled extensively. Emily wore a size 6 and lived by the rule, you can't take it with you. She spent a fortune on tailored clothes and lingerie.

It was in the early '60s, shortly after we'd bought a condo in Manhattan, that Emily demonstrated the first sign of Alzheimer's—though no one knew it at the time. She was caught shoplifting at Macy's. I couldn't believe it; she had all these charge cards and I'd never denied her anything. Emily said she just got mad standing in line. We never discussed it again.

We wanted to live abroad, and in 1972 we moved to a waterfront condo in Majorca, Spain, where we could look out the window and see the King's children playing on the tiled walk. In the five and half years we were there I had to fly back to America 27 times to be an expert witness in electrical engineering lawsuits. Fortunately I was back when Emily suffered her first fracture. She fell down in my lab, screaming in pain. We found out it was osteoporosis, which eventually led to multiple fractures of the spine. Soon we decided to move back to the States, and I suggested Fort Lauderdale.

We loved our Florida condo from the day we bought it in 1978. Emily was gregarious and made friends in the building, but her memory was starting to fail. She had a hard time remembering simple things like where she was going or what happened yesterday. I insisted she have a CAT scan checking for a brain tumor, but nothing showed up. After further tests, they diagnosed Alzheimer's. I kept all this quiet. Didn't even discuss it with Skipper or my friends. It's not my way. I kept it all inside.

I set up a shop in the apartment and consulted to companies up North, but Emily became more and more dependent. If she saw me leave on a business trip, she'd throw a tantrum. I tried to make day trips or leave at 2 or 3 a.m. so it wouldn't seem as if I was gone overnight. That went on for a couple of years. But her fear complex grew worse by the day. Friends tried to help out, but she only wanted me. In 1984 Emily woke up and had a fit because I was gone. She remembered nothing. She ran to the neighbors demanding to know where I was. "Did he leave me? I can't make it without him. Find my husband." When neighbors reached me by phone and she heard my voice, she calmed down. But I came home at once.

For months leading up to the end, our living ritual was virtually the same. I would get up at dawn and get us two glasses of orange juice. She slept on the couch because it was more comfortable for her back. I'd sit down in front of her, sip my juice and wait for her eyes to open. Usually she didn't know me. She would ask, "Where are we? Are we in Spain?" It was pathetic. I'd talk to her and try to bring her around.

I'd pick her up and carry her into the bathroom. I'd bathe her, brush her teeth, perform her feminine hygiene for her. We would go out for lunch and she would pick at her food. She had no appetite. She weighed less than 80 pounds. There wasn't much I could do except make sure she took her daily medication. I asked the doctors for help, but they had little experience with this condition.

The weekend before the end was awful. She was in such pain from new fractures I took her to the hospital; the doctor said she needed to be X-rayed. But she got recalcitrant and carried on like a sick baby. She refused to take her clothes off for the X ray, and when they tried to take a blood test, she yanked the needle out of her vein. Blood flowed all over the bed clothes and her already soiled dress.

I stayed in the hospital all night with her, but she refused to stay in bed. The pain was so bad she screamed and cried. She went out in the hall looking for elevators and pushed all the buttons, which caused an uproar. The nurse said, "We can't handle her; we're not used to this disease." She echoed the words of three nursing homes in the area, which said they were not equipped to take her in.

So we left on Sunday. I put her back on the couch, and she hadn't eaten for 36 hours. I gave her Percodan. What else could I do?

The next day I had to go down to the manager's office and sign some checks for the condo association. After a few minutes she must have noticed I was gone. She hobbled onto the elevator and came down into the lobby. She was dressed in the same bloody dress that she'd worn in the hospital. She looked a mess. She was crying. She brightened up when she saw me, but then I started crying. And the office secretary started crying.

I took her back to the couch. I just looked at her and cried and asked myself over and over, "What the hell can I do?" She said, "Ros, I love you dearly. God, I want to die." It wasn't the first time she had said this. I even thought about killing her a year before. Had she not asked me in those last few hours, I wouldn't have gone through with it.

I walked into my lab and fetched the Luger. I shot heron the side of the head. Then I did something foolish. I felt the arteries in her neck. I realized she was still alive. So I reloaded the gun. I was shaking like I'd never shaken. I shot her again. Her chin dropped. Her mouth just opened. Her right hand shook. No other movement. She was dead.

I felt grief. Not regret. I stood there and cried thinking my wife was dead. But the fact that she was no longer suffering gave me relief. The prosecuting attorney later said I got sick and tired of caring for her. He had no right to say that. Emily was terminal with both diseases. I did what I felt was right.

I didn't give a damn for the trouble I faced, but worried that Emily's sister would be angry. She has a heart condition. I called later and told her what happened. She was upset but she understood. I didn't dwell on it with her or my daughter. It was my problem. I've always been alone, being an only child. And I deal with things my way.

At the trial people said I was cold and didn't show compassion. But they didn't know the turmoil was raging inside. I was tense, nervous and constantly battling, but I hung on to my guts. I've spent too many years as an expert witness, and I used the same factual, objective approach this time. When they asked me why I killed my wife, I said I had no moral choice. Emily had lost all capability of objective thought. Most of the time she didn't recognize me, my daughter or our grandchildren.

My manner was wrong. But if I had to do it over again, I would do it the same way. You are what you are. No testimony allowed would indicate to the jury what type of man I really am. I'm a scientist. Dammit, I had no alternative.

I do know this. If I get out of jail, I intend to work to legalize euthanasia. I may be helpful now that I'm well-known. I got caught in a bad law. Compassion has to be part of the defense.

Emily said she wanted to die. Through our life together I loved her candor and loyalty. After stating her opinions, she always allowed me to make the final decisions. She thought I could solve all problems of life. She thought I was God.