Okay guys, I'm not sure how much bearing this has on how many bunnies you have to kill to get the killer bunny, but I only had to kill 20 to get the killer bunny. I had picked up and read Robin's hunting log, which mentions that he had killed 19 bunnies, and had heard rumors of a "killer bunny", but never found one.
I, too, waited till I was in the bandit camp (though it was my first visit) to kill the last bunny, then hopped on top of a sort of roof over a feed trough near the hogs. No one aided me, and I got a lot of experience points for killing it. I might have been level 12 or so. I also got a few Elder items, great stuff for so early in the game.
PK