Regarding Nero's motivation... It was simply a very misdirected revenge. He blammed the Federation for not being fast enough to help his planet (as if the UFP *had* to help Romulus...). His ship was simply a mining ship, but overpowered since it was from 130 years or so in the future. The black hole technology was from Spock's ship. Remember Nero was pulled into the past unwillingly. However I wonder why he waited 25 years roaming around in space, in the hopes of getting his hands on Spock and his blackhole technology to execute his irrational vengeance..instead of going to Romulus and help them prepare for the upcomming Super Nova. Or even use the black hole technology for that purpose instead...
1. his ship wasn't a simple mining ship. It was a mining ship turned into a superweapon thanks to it being refitided with Romulan adapted Borg technology.
Info from the Countdown comics.
2. He wasn't just roaming 25 years in space. After the Kelvin incident he was captured by Klingons and spent quite some time in Rura Penthe from where he later escaped.
This plot was cut in the editing room
3. As for Nero...I suppose one can be expected to be half mad and irrational when one sees child, wife, entire planet destroyed and spends a few years in a Klingon prison