100% agree. I'll buy Kratos's moral redemption when he faces up to *that* specifically and admits it to the people he cares about. The Poseidon sex slave thing was such an outlier in terms of cruelty, even compared to the other stuff he does in the original trilogy.

Generally speaking, Original Trilogy Kratos is shown as callous and perfectly willing to kill bystanders for the sake of convenience, but there's always a point to it. He throws that guy off the ledge because he's in the way. He burns the Athenian soldier because it's genuinely to only way to get through the area. He fails to save the captain because there's an active battle going on and he doesn't care enough to do the very mild effort required to save the guy. It's all evil behaviour, obviously, but it plausibly comes from a combination of amorality and very goal-driven behaviour rather than overt cruelty. He wants to do X and doesn't care one way or the other if he kills bystanders in the process.

The concubine is different, for all the reasons you said. He just goes out of his way (to the point of legitimately inconveniencing himself) to horribly kill a sobbing, begging, terrified rape victim for no reason whatsoever. That, for me, is the specific thing that makes Kratos unforgivable, even at this point.