Probably most famously, Woody Harrelson played psychopathic murderer Mickey Knox in Natural Born Killers.
Whoever plays Carnage and his alter-ego Cletus Kasady has to be able to sell insanity, and Woody Harrelson has proven he can do that.
In response to a Facebook question about the casting, David Michelinie notes that Woody Harrelson has played “nutso” characters in the past, and that is an understatement.
Venom is traditionally a villain in the Spider-Man mythos, so to make him look like the good guy, the villains have to be truly malicious, and Carnage fits the bill.