According to Star Trek: Unity (fan film series) continuity, although appearing human, Nero was in fact a Neronian leader who decided to invade Earth for his own purposes. Instead of using military might, Nero, fascinated by Earth's Roman culture, decided to make himself Rome's leader. He left Earth in 68 CE, however, when he beamed away to his ship after an uprising and made it appear that he had killed himself with a knife.
Nero retreated back to his empire and entered frozen animation to wait for the Neronian armies to build up to a point where he could invade Earth as he had always wanted to. He was killed in mid 2383 when the USS Hawk-A destroyed his mothership.