"This is my sister kim, the bitch from Gone Girl," Rob, 28, captioned a photo of Rosamund Pike as Amy Dunne in Gone Girl, covered in blood.
However, it seems there's no cause for alarm. "He has a funny sense of humor and posts weird things and then deletes them," a source close to the Kardashian family tells PEOPLE. "It's just him."
Indeed, as a source previously revealed to PEOPLE, nobody is particularly bothered or insulted by the bizarre slam, calling the whole thing "so dumb."
That said, some of the family are concerned about his behavior. "He's been depressed and just a bit off in general," says the source of Rob, who has struggled with his weight. "But he has always been this way. He just can't lose the weight and he doesn't try – he just sits around."