I think Willow would top my list, too. I mean, she flayed a guy alive. She also shot him in the chest -- slowly. That was a gruesome, premeditated death. He was murdered out of vengeance. If Faith had to go to prison, then I think Willow should have had to do some time, too.
Really, though, all of the characters have some amount of blood on their hands. Even sweet Fred, who killed the professor (with Gunn's help) was no innocent. Some characters' actions killed inadvertently or accidentally (for example, Xander summoned Sweet). Other characters did so out of necessity; in war, there are always casualties. One dead guy is better than an apocalypse. That's why Giles killed Ben. I was on board with that. Some people were killed or otherwise dispatched in self-defense (hyena man and Amy's mom would, for me, both fall into that category). Very rarely, though, core characters' actions were premeditated and/or motivated by a desire for revenge. The murders of Warren and the professor both fit that category for me. The murder of Lindsey... locking the W&H folks in a wine cellar with Dru and Darla... these things didn't even fit into the gray area for me. Those are the types of actions for which I don't think Buffyverse characters deserve a get-out-of-jail-free card.
