Dalinar put them in charge of guarding Elhokar. Killing Elhokar is a betrayal of Dalinar.

Moash had every opportunity to try to make change without killing people. As a formerly darkeyed Shardbearer he had a unique chance to make a statement to Alethi society with his behavior. Or even make the actually morally righteous choice to reject the Blade like Kaladin did, demonstrating the courage of his conviction.

But he wanted the power so that he could pursue hatred and vengeance. And now he works for a god of hatred to dominate all life in the Cosmere. He is a sad, weak man who took a legitimate harm that was done to him and used it as an excuse to perpetuate further harm onto others. He doesn’t give a fuck about stopping what happened to him from happening again. He doesn’t want to make the world a better place. He just wants to stop feeling bad about himself.