I kill flagged toons within my level range just once on principle, because it's good PvP mechanics, especially for those quests that automatically flag you, e.g. Grizzly Hills and Ymirheim. If you're doing a PvP quest and are in no danger of being killed by an enemy player, then the quest isn't working as designed (on the same note, even as a carebear I find it annoying when people rant about people killing them while fishing for the achievement in an enemy capitol. It's not SUPPOSED to be an easy achievement, that's why they put it in a ridiculously dangerous place!).

I don't kill anyone twice unless they attack me. I won't kill a player a lot lower than me even once, but just give them a menacing /wave and be on my way. PvP mechanics are one thing, bullying is another. You can assume that someone who just can't stand up to you but is still flagged is probably not still asking for it -- it takes awhile for the flag to burn off, after all. I don't want to grind someone's play session to a halt because people have done it to me and it sucked the fun out of the game, so ... golden rule, an' all.

I had a bad time in Grizzly Hills a few weeks ago where I was five-manning the shredder return quest, and an Alliance DK realized that, for the price of a guaranteed corpserun, he could kill my shredders 100% of the time before my caster-heavy group brought him down. As ridiculous as it sounds that a single toon was griefing my 5-man group, that's pretty much what it was. The first time was funny and kind of admirable, one dude kicking my ass. /salute, y'know? It taught me to bind a key to "leave vehicle" if nothing else. The second time was a little obnoxious. The third time sucked ass. My solution, and it's probably not the best one, was to grief this little Alliance warrior trying to do his version of the quest. I killed him twice, figuring he'd bitch to the DK to stop messing with me and just let me finish so I'd leave. I don't know whether it worked the way I planned or if the DK just got bored and left, but I delivered my last shredder without issue. A small stain on my honor, but probably not by many peoples' standards.