I don't think he's said, but he has written some shockingly creepy stories himself, like the Halloween special which suggests Jon died or moved out ages ago and Garfield is just hallucinating due to starvation and despair. He claimed he wrote it after a market survey indicating that loneliness is what people fear the most (this is a pattern with Davis - he always cheerfully claims he's just in it for the money whenever someone suggests he has any kind of artistic vision).

Or the one in "Garfield: his 9 lives" where a different incarnation of Garfield goes suddenly feral and kills the elderly woman owning him. Jim Davis didn't draw it, but he did script it!