I actually killed a squirrel on my bike a few years ago. I was bombing down a hill, having fun, when a squirrel chased another across the road right in front of me. I tried to brake, but it was too little, too late. I caught the chaser under my front tire, right at the neck. I heard a little popping sound, and knew it was the end. I slowed down and looked back, and sure enough, there was the little guy, dead as a door nail. I felt pretty bad about it, but I imagine the squirrel he was chasing felt kind of relieved at the time...