Omni-Man has been a hero for most of the Invincible series.
A lot of people consider Omni-Man to be the best Invincible villain, but surprisingly, he’s only been a villain for the first 12 issues.
After that he began his redemption arc which started when he went to a big planet where the aliens have a very short life span.
It was there that he learned that it’s not about how much time you have, but rather it’s about what you do with the time you DO have. He even had a child with one of those aliens.
After that he reconciles with Mark and does various things to make amends, one thing led to another, and through all that, Omni-Man has been doing his best to be a hero.
It’s a shame we never see Omnimans history as a young man on Viltrum. If he is a true believer I’m sure he killed thousands of innocent Viltrumites that were either too weak or didn’t believe in the empire.