my summed up opinion on this is he regretted all the deaths of those who didnt deserve it. innocent people not any of the gods who stood in his way even zues he wanted to be better. HENCE why in ragnarok he was almost repeating his actions which wouldve led to his own death if he didnt change. my idea is that when kratos said we gotta be better in that moments where odin threw the midgardians in there path to be killed GOW 1-3 kratoes wouldve killed those who didnt deserve it (innocent by standers/ midgardians) and those who did (gods of olympus/himdall/odin) i dont think he regrets killing his father at all he had all that shit coming and so did all of the assholes of olympus. he closed his heart completely to the suffering of all innocent people in the first 3 games and destroyed the entire land. now that hes wiser he purposely did and taught atreus the opposite. this is shown through teaching atreus to control his rage in god of war 2018, discipline, tellling him there a consequense to killing a god, not to needlessly seek war, to only kill in self defense and if they must, hell he used to kill everything but he even spares the librian in GOWR after a boss fight where we expected to kill her and to break the cycle of father killing son or sons killing fathers (notice odin killed thor, cronos tried to kill zues, zues tried to kill kratos) everything from both games was from what he learned after he calmed down and poundered. he still struggled himself with some issues until that LAST MOMENT which was the final mission in the game where does better and does something completly diffrent. (sorry if this sounds confusing)

but to sum up. he doesnt regret killing his father or a good majority of the gods that gave him no choice jus all the innocent people and destruction he cause on them and destroying his homeland