[Garak]He assumed that because that's exactly what the Romulans would have done...[\Garak]
That's probably it, combined with a mistrust/antipathy for the Federation for not making the Vulcans help prevent the mess in the first place.
Well, let's not forget that the Romulans weren't exactly fretting over the whole thing, either. There's blame all around. Starting with the Romulans themselves. That's why I agree with Rat Boy.
Also, why would Nero think the Feds want Romulan space, when that's exactly all that's probably left there? Space, no planets. Further, the Feds have enough to worry about if this star has started to do what Spock said it would do than to make a territory grab. Didn't the Romulans learn anything about the Federation during the war with the Dominion?
The Romulans have a long and historied past of distrusting the Federation. The peace was not easy.
Also, it was heightened in Part 2 that the Romulans and Vulcans generally didn't trust each other, conditioned by 200 years of bad blood.
Also, Nero just saw everything he knows and loveds destroyed, and the Vulcan Council rejecting his plea for help.
That kind of anger will bring out the distrust. He's not thinking logically, he's thinking with his base instincts: Paranoia and Anger.
He's probably going to lash out at the Romulan senate as well, since they ignored his warnings.
When he sees the Federation as the cause of the death of everything he knows and loves, he's going to take revenge, and destroy them.
He has absolutely nothing to loose by doing whatever he wants.