I kill them all, anytime, any situation. I will occasionally let someone live, but only if I can induce more interesting pain by doing so. For example, I might help an alliance complete a group quest, but then I'll camp them so they can't reach the NPC and turn it in. I find this behavior generally brings out L70s, and then I get a good PvP workout.

Generally speaking, I will kill someone then ride off in a different direction from the one I'm actually headed. I find this keeps me safer than being a "nice guy" and letting folks follow me around and emote goofy things, etc. Curious folks have guildmates, and not all guildmates are of the watch and admire quality.

Occasionally this behavior catches up with me, but so does letting people live and attracting attention due to "Wow, you gotta see this!" comments in /g, General, Trade, LocalDefense, etc etc...

I'd kill a level 4 Gnome with 10% health that was busy fighting a mob... happily, in fact!