It's a bit of a superstition that I developed after a beloved pet (a rabbit) died in real life shortly after I'd killed a rabbit in game. I should point out that I'm fully cognizant of the fact that my killing a digital rabbit in no way, shape or form actually harmed my real life bunny. It's simply not something I want to do ever since it happened. I don't want or need to believe in any sort of causal link to simply find it unpleasant to hear their little death screams in game.
I've gone so far as to kill a flagged player I had no interest in PvPing beforehand just because he killed a squirrel outside the Darkmoon Faire grounds.
I also generally won't gank lowbie players. If a toon isn't within at least 3 levels of me, it's just not sporting (as far as I'm concerned) and I don't see the point. I actually even rolled a horde toon on my old alliance server back in the days of Crossroads raids and geared him as best I could in BG gear just so I could attack my own guildmates (and let them know I was doing it) for raiding XR.
So now, how about you? What won't you do? Run lowbies through instances, twink, roll a warrior, what is that thing in game you're just never going to give a try?