Curate, connect, and discover
I'm rewatching the episode, and Belos is SO deeply fuckedupedly religious, holy. This is ALL about Institutionalized Christianity. No wonder Disney cancelled this show. Promising myself to not make this too long so here's some quick points:
Humans are born into sin, inherently sinful and must spend their lives repenting for their nature (simply by being 'witches' he probably believes there is no such redemption for the inhabitants of the Isles anyways).
Your soul will be doomed to the ashes of hell if you don't repent now. Belos probably saw the 'devil' in his brother and thought he was doing a service of god to kill him.
Remember we are dealing with The Demon Realm. Belos probably believes he is trapped inside some kind of hell. The Titan is the BI's version of God, and he just repeats the rhetoric he's internalized to make it digestible for the citizens here.
He literally thinks he is a saviour and all of the carnage is worth it because he is ridding the world from the influence of the demon realm; which I am sure is very intentionally a society free of homophobia and racism, things that have been largely installed by Christianity and religion.
I might go so far as to suggest that Hunter's grooming by Belos (which doesn't have to be explicit in that way) is probably in reference to the proliferation of grooming done by priests, often with the excuse of this is 'how god wants it' / 'the titans plans'.