I mean the crab people stopped fighting as soon as the the creature Arthur summoned showed up as it leveled half their army, a good portion of Orm's army was rendered useless because Aquaman controls the sea creatures they're on, and then half the army immediately stopped once Mera explained to her dad that Arthur had Atla's trident. So they're were plenty of ways for you to avoid some of the conflict and unnecessarily killing so many Atlanteans.
Of course some of it couldn't be avoided, Orm immediately ordered his army to attack whatever that creature was, and they use a lot of tech in Atlantis that couldn't be stopped without force. But that creature tore through the army like nothing and seemed pretty fine. There was no need for Arthur make all the sea creatures eat the people riding them or to summon the trench people, which just leads to more death. Either way, we're supposed to believe Arthur is ready to be king of Atlantis, but he doesn't prove as to why he should be. The only reason he is, is simply because he acquired the trident. He didn't learn anything. His first plan was to go guns blazing in the finale, much like it was when he first arrived to fight Orm.
Heck, as half-baked as it was for a side plot, you could try and argue Arthur's whole encounter with Black Manta was meant to teach him that chaos and death isn't how you should handle things, which makes it even worse that his first instinct was just to blow everything up.
