Squirrel.. the little bastard kept chewing holes into my roof and got into the eaves. I was afraid he was going to start getting into the wiring. I started with sympathy for the little buggers, and tried to deal with them nicely. Towards the end, I would see red and feel murderous anytime I'd see a squirrel in a park.
I almost hit a cat with my car on two different occasions. But I avoided them just in time. So unless I've hit a mouse or something and didn't notice, I'd say a spider is the biggest animal I've killed. Though in my defense, one spider I killed had a body the size of a US nickel and its legs stuck out to make appear even bigger! While I have nothing against hunters and I'm certainly not a vegetarian, that's just not something I want to do!! I love animals too much (unless they're creepy spiders).
When I was younger a kitten we had got a cut on its side. My dad put peroxide on it and the cats guts fell out. My dad called a vet to see if there was anything we could do and he said the cat was probably not going to make it so my dad had to put it down. I felt pretty bad :\. Other than being there for that one just bugs and stuff.