This was only horrible after I realized what I had done, and immediately made it so my character didn't know what she did. I was playing a female and writing a story about her adventures, though it didn't get too far. Up until this point, she'd been fairly innocent, killing gophers to get stuff to sell, and fawning over jewelry and fine weapons in the shops. The closest thing to bloodlust that she got was chasing after a wounded alligator until it turned around, knocked her over, and kept retreating towards the river.
Then she was sneaking up on a couple animals, and a Rhesus Macaque(which fills me with hatred) walks up to her and breaks her stealth. She threw a couple rocks at it, cut it twice, and then put her sword away when it gave in to the pain. Then she grabbed its head with her free hand, ripped out both eyes, bit it in the neck, and shook it violently until it died. A method that I use frequently with characters I don't care about. Then I realized I had to write about her doing that, and how she felt.
So the worst thing I've done has nothing to do with destroying towns and civilizations. It was violating my own character and then realizing what I'd just done. That's recent though, and I've never been talented enough to wipe out towns anyway.