I think all the execute skills should have some sort of secondary effect like Mercy Stroke. Something like;

* Assassinate: If enemy dies within 3 seconds of use, cooldown is refreshed.
* Misery's End: If windbite/venom active on enemy and they are below 10% HP, they instantly trigger for an additional 50 potency each.
* Heartbreak: If enemy dies within 3 seconds of use, Quick Reload cooldown is refreshed.
* Mercy Stroke: same thing, but changed to proc within 3 seconds of death instead of having to be *THE* killing blow.

That's just me though. I don't like when all skills are the same.