It's not just the amount of prior hits.
It also depends greatly on the force of the impact. If it's hard enough, and to the head, then one hit is all it takes to kill (and all it should take, really).
That's why drop-kicks to the noggin are deadly even if it's the first hit.
And drop kicks to the head do take some skill (especially if you drop kick with a cliff behind you like on broken arrow, a drop kick risks you falling off the platform yourself) so I think it's appropriate for them to do that much damage if done to the head.