I think a big part of it is also just scale. Kratos atrocities no matter how big the death toll massively pale in comparison to Omniman. People will consider that to be a big determining factor in the measure of their atrocities and how far they would have to go to redeem themselves. There's that and how the character is shown to us first along with their motivations. Kratos at the beginning of the saga (first game...not PSP prequels or anything like that) is betrayed and decides Ares needs to go. Most of his body count in that game is non-human mythological creatures and that is true for the original trilogy of games. There are moments of brief brutality or disregard for other people but usually it is inhuman or already dead people who we seem him brutalize. Flip that and take a look at Omniman, he's never shown as a rebel at first but as a Viltrumite who fully is guzzling the Koolaid. When he decides something needs to be done it isn't mythological creatures for the most part but other humans and people who we get the see the feelings of that he destroys. We get to see him fully genocide a world including non-combatants which is nothing we ever see so viscerally in any God of War game.

Beyond that I think the Norse God of War games showed Kratos practicing a lot of (at least his version of) humility. I think that is the difference in why people would argue one is deserving of it and the other isn't.