Instead of wiping out the human race, I think a far crueler punishment would be to make them immortal, but never heal from their wounds, yet still feel pain, and continue living no matter what injury befalls them (even fatal ones). Considering the average injury or injuries an human being acquires on a daily basis (the most mundane to a small cut/graze from a corner or walking on rough surfaces, or a bruise from walking into something), and the fact that such pain would stack overtime; have fun "insensitive" immortals. Even in a week or few, you'll be living in eternal misery for the rest of your life; which you can't end either, and suicide would only make it worse since you can never actually/entirely die. Severing limbs won't make things any easier (to sever a connection to a bigger collection of accumulated pain) either since you'll have the cut point as a constant pain point (and receiver since it's freshly sensitive after exposure as well) as well.
Oh, and as a contingency plan, the brain can also feel pain now; just in case you intend to cut off your pain receptors or motor nerve clusters.
Man, considering such a punishment, death is actually a blessing or an act of mercy, by comparison.
Back on topic, yeah; What the hell, Humanity? (And I'm one to talk, with that punishment above. :\ )
EDIT:
[derail]
This is kinda why nothing scares me or shocks me anymore. Horror movies only bother me, rather than entertain or scare me (unless it's an actually good one like The Thing (original and James Cameron)). I mean, on this tangent, if I were in a horror movie, I would know the quickest and most efective/efficient way of out-pacing the bad guy, and actually taking his place (by being more legitimately terrifying than them) until they learn their lesson. And that's when I'm being merciful. Without mercy, I would invoke the example above by request to God (or my own means) whether I get offed by the bad guy or not, and maybe come back as a zombie as well to add to the fear factor if I were offed.
[/derail]
EDIT EDIT:
Sorry, rapid-editing again.
Other things I would be doing if not merciful... well, just look at the many many things I listed in the Bay-12ers Not Allowed to do in RPGs thread. I define Complete Monster there, and even I admit it. I mean, in case I ever do go to Hell (God forbid), I have a few acres reserved for a nice little cottage and demon-cattle ranching or something. Or at least, a broodwich deli-ingredient provider.