@Acolytus silly speedreader, didn't they say that the merfolk prince got killed by humans after trying to get peace (or something like that i don't remember)?
He feels bad because merfolk attacked humans with a good justification and as humanity protector he had to kill the invaders.
That's why he circled the merfolk city with Jormung whatever , so that humans don't fuck with them again and he also swore to not meddle with them as long as they don't genocide.