Nero wasn't going anywhere. The Narada was falling apart, pieces were falling off. It was on both sides of the wormhole, not going in one side and coming out the other in some other reality like we saw earlier.
We don't know that. We do not know that at all
. The Narada
was at least a century more advanced than anything the Federation had -- if not more, if it was made from Borg technology -- and if it survived one trip into a new timeline, there's no reason to think it can't do so again.
And do recall that Nero declared his intention to continue combat with the Enterprise
if they attempted to rescue him.
This is akin to capturing an enemy solder who cannot run away, who has no weapons left and shooting him in the back.
No, it's akin to shooting an enemy who's about to walk into a village with an AK-47 and shoot someone.
It's not surprising the support Spock has for his actions. How many war crimes have gone unpunished in Iraq and Afghanistan? If you take on the same actions of your foes, you become them.
Dude, killing your enemy in a firefight he started
when he has not yet surrendered, has not been captured, and represents an immediate threat to innocent life, is not a war crime.