Another point that someone mentioned why they might act different is because these are the "blue-collar" Romulans. So far a lot of the aliens we meet on Star Trek (Romulans included) are either diplomats or soldiers.
Nero and the others tattooed themselves because they are extra pissed off about their planet dying. Besides, Nero's all screwy in the head so who can ever ascribe his actions to any more detailed logic?
Romulus blowing up was sadder for me than Vulcan, because Romulans are cooler than Vulcans.
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