Come on Zeni the skill is useless. It NEVER heals you unless you are attacking boss mobs. It needs to work like siphon does for the NB or it's just a waste of skill-points. And even then it won't heal even half what Siphon does.
Yes it only heals if it runs its full course, which makes it completely useless. If a mob dies while still under the effect, no heal. I'm assuming that's a bug or an oversight, but it needs to be changed.