I don't mind PKers like Shiirn, because I ask people to stop attacking if they're annoying me. If they kill me, I'll get a bit annoyed, but I usually talk it through with them.
What pisses me off are people who don't listen and keep repeating the same mistake. That's when I think a PKKer is needed.
Even though I agree with Shiirn on most of his/her points, I don't like random killing sprees. It's true that a good fight can be started by a spree, but most of the time you just end up angering random people, which isn't all that nice for both sides.