Backloading Dark Harvest's damage is a good idea, but just "half health" isn't backloaded enough.
When your aggressive kill-focused rune is best used by
and company, you know you've got issues.
I really do think you're going in the right direction, but you need to take this idea further.
What I'm thinking is that you make Dark Harvest an execution threshold of sorts.
"If you deal direct damage to an enemy champion such that an additional X (+X AP) (+X b.AD) would result in their HP being dropped to 0, Dark Harvest will deal that damage. This rune gains base damage equal to Y times your kill count."
Imagine Electrocute, but instead of a burst that takes your target from 70% to 60%, it's a burst that takes them from 10% to 0%.