Oh, I find Nero's behavior much more comprehensible, especially post-PIC S1. To his mind, the Federation promised to help and then stabbed Romulus in the back. As far as he's concerned, Spock was probably using Red Matter to make the supernova happen sooner. He is being irrational, yes, but it's an emotionally comprehensible irrationality -- his anger and rage are comprehensible and his reasons for targeting the Federation make sense as a combination of "they stabbed us in the back" and general bigotry. I can totally see him wanting to wipe out the entire Federation in revenge, even if it's the wrong century. "They're all the same" etc.