Nero murdered 6 billion. Kirk Prime (and Chris Pine's is just as "real") never came up against ANYTHING like that - not from anyone accountable (i.e. not the doomsday machine)
Khan had every intention of killing billions with the genesis device, kirk still did not blow him out of the sky when the reliant was a sitting duck. That is the character of kirk, and the mark of a good starfleet officer who practices the principles espoused by the federation.
I'm lukewarm to Star Trek 2009
is fecal matter to be perfectly honest.
We have no idea what Khan planned to do with the Genesis device. And comparing the Kirk's really doesn't cut it, we have no idea what Prime Kirk would have done with Nero. We have no idea because the situations are different, Reliant
was trashed and not sitting on a possible escape route when Kirk's help was refused. Khan had not just committed genocide and had not killed Kirk's father.
If you can't see that Kirk made the right decision in regards to Nero then you're just allowing your dislike of the movie to override your common sense.