Spock Prime and Nero are two men out of time. The movie doesn't attempt to tell their story, beyond the mindmeld which gives a small glimpse into why they are there. The movie isn't about them. It's not that there aren't answers to the questions surrounding Nero, it's that those answers are left open. Just like the Whale Probe, they don't bother telling us the first thing about that probe, other than what we need to know. It's there to look for the whales, and for some silly reason is willing to destroy the very ocean the whales would inhabit because they haven't phoned home in awhile.
That doesn't mean it doesn't have it's reasons, it's just that they are left open for us to ponder. Nero is the same way.