You aren't paying attention. The trigger is "drops to 0 hp". At this point the attack has hit and the damage has been dealt. The only thing that can be invalidated is the state of the orc's hp, which can only be accomplished by healing. Killing the PC that hit him won't do a thing: the attack has already landed and can't be prevented, and the damage has been dealt and can't be reduced.
Actually,
you're not paying attention and obviously didn't bother to click on the thread he linked - that discusses an almost exactly identical scenario, except rather than triggering on dropping to 0 or below, the interrupt in that thread triggers on becoming bloodied.
Basically, there is no such moment between a hit and damage - the two are interlinked; just as there's no such moment between leaving a square and entering another (similar discussions have been had on that topic too). You can interrupt seperate squares of movement (explicitly stated), separate attacks (fairly clearly, but no explicit) and seperate actions (almost by definition, though even here, for No Actions and some Free Actions, it's arguable). As far as I can tell, that's it; there's no rules basis for interrupting half-way.
If you rule that interrupts can split events half-way you're opening a
huge can of worms. What actions and events can you interrupt half-way? Must you do so, or can you choose to interrupt the "entire" event? Is
any of this defined or even ever
mentioned in the rules? At which points half-way can you interrupt?
For example, Shield triggers when you are
hit by an attack - why isn't that
after the attack roll is resolved? Polearm Gamble triggers when you you enter a square - why isn't that
after you leave the previous square?
I'm not saying that when the author of the monster brainstormed about what an orc reaver should do that avoiding the attack was the aim of the power. However, using an interrupt is an easy way of writing the power; and they
may have been fully aware of the fact that this power does a little more than merely grant an extra attack - and chosen this (simple) wording rather than a more complex approach. Or maybe it was just an oversight, or intended all along - I can only speculate.
But I do know that splitting "getting damage" from "losing hitpoints" leads to all kinds of weird situations; and if you can split that, what can't you split? This is clearly not a reasonable reading of the rules.
So, particularly since this is a monster power, a DM should do whatever works for the versions of the monster that occur in his game; and RAI may even be that the orc drops despite interrupting the killing blow. However, a DM shouldn't delude himself into thinking that because
this monster with
this specific power works best and most naturally when the power is resolved in a kind of limbo right before death that interrupts should
generally work this way.
In this specific case, both interpretations will be eminently playable. A DM
should fit monsters' rules to his desires, particularly in cases where it seems the power is simply poorly worded. But do not extrapolate how interrupts work from one possibly wonky power.