Personaly, while Donovan is still a very f*cked up character, I think Wharton is much worse for two reasons. First, Wharton raped two girls and then killed them and (as it's revealed in the book) bashed their heads togheter to kill them, Donovan never killed Guts and there is no proof he raped anyone before him. Second, while Donovan has a similar body count to Wharton, he killed two children and a pregnant woman, while Donovan's kills were just male adults.

Not to mention, Wharton may have been killing even before The Green Mile.

That too.

Also, Donovan is a mercenery, so him killing soldiers during the war isn't even a crime.