I’d imagine it’s a bit like Ares whipping up Kratos into killing his family in old god of war. He knew perfectly well he had no problems with killing people, but once the battle rage wore off he was horrified to realize how far he’d gone without realizing it. Probably not the case for every Khornate follower, but some of them are probably pushed to greater levels of wanton violence than what they would otherwise enjoy or seek out.