Timing is everything. You need to wait until the player takes his attention off of you. Most people are familiar with it, so they no to make sure a warlock is really dead....but they don't know exactly how long to wait unless they ARE a warlock. So I've only had one player kill me a second time. I'm the one whose usually handing out the revenge killing. Because I'm patient enough to wait until the right moment, rather than just angrily doing it right away.