The problem I usually face when asking the question of what the most gory PC game is is that I've basically played every sick, twisted, gore-filled game.
I've played both the Manhunts (Manhunt 2 on PC is UNCENSORED!), all the Postal games, all the Soldier of Fortune games, The Punisher and most other titles that have a fair share of gore in them.
I'd recommend Manhunt 2 on PC for anyone who really enjoys violence and gore. I've played through it twice on PC and Rockstar Games was gracious enough to provide PC gamers with the uncensored AO version of the game.