The blow that;
A. Actually takes the target to dead
B. Takes the target to negatives and dying
Imo, B.
The blow that results in the enemy acquiring the "dead" condition.
There's no rule-specific definition of "killing blow", so use the plain English definition.
Technically Bjorn and Blaphers are correct, but I agree with Valandil.
If you reduce someone to -1hp and then they bleed out you've dealt the killing blow.
If you reduce someone to -1hp and they stabilize, and then you leave and they wake up later ... well you'd think they were dead and it just creates more work for the GM.
If the other party members beat everyone unconscious and then you walk around and Coup De Grace 100 unconscious soldiers it technically fulfills the "killing blow" requirement, but isn't really in the spirit of the feat.
My opinion: "Killing blow" = unconscious, but if you capture and release then it doesn't count.
That's a reasonable interpretation that still respects the plain English definition. So long as they die as a direct result of your hit, it shouldn't matter if it takes a few turns for them to bleed out. Similarly, if someone else stabs your foe until they're unconscious and dying and you finish them off before they bleed out, you delivered the killing blow. (This goes both ways--if you stab someone to dying and your friend finishes them off, they delivered the killing blow, not you.)
Technically, it would count toward the feat's goal even if the foe dies but is later resurrected. That doesn't mean you can just kill-and-revive repeatedly, though, as the goal calls for "a number of your hated foes"--killing the same one over and over still only counts as one foe having been killed.