If you read his bio, it explains that Let Volir joined Atriox out of necessity despite his contempt for Brutes; he needed the supplies for his ship and soldiers.
That might seem at face value quite a nice trait. But, it ignores a few big problems.
He is described as not believing in the Covenant religion so he didn’t want to side with Jul M Dama (fair enough). So I was thinking, “wait, why doesn’t he just join the Arbiter?”. Well, yeah, he apparently hates humans so much that the Arbiters policy of not killing them on site is offensive to him.
Few things:
Firstly, the Arbiter is awesome and hes a fool for not joining him. 
Secondly, you’re deliberately putting your own men at risk so you can continue this war with humanity? Which defeats the object of saving them. That just puts them in danger by slaving them to the whims of an ambitious warlord.
Thirdly, why do you hate humans if you don’t buy into the Prophets religion. The only reason Covenant races want to kill humans is because of the Prophets decree that they should be killed and this was the will of the gods. Its basically counter intuitive.
Which leaves the only remaining answer that he hates humanity, not for any sincere if misplaced ideological reason, but because he really is just a alien fascist out to increase his own races power at humanities expense; thus profiting from the suffering of others. He is a mercenary, hes a turncoat and he has no honor. I mean at least the old Covenant elites were sincere and honest in their beliefs; warped by the Prophets. This guys just a creature, hes got no principals or morals.
What do you think? Don’t get me wrong, I think all of the Banished Commanders are cool. But this guy is clearly a bit of a monster when you scrape away the pretense of looking out for his crew like some pirate lord. I am fascinated by how the Banished are much more rooted in power and self interest than the Covenant which was an army of Crusaders. I mean I hope this doesn’t mean we’re never going to have the Crusader army again (who I suspect will end up joining Cortana).
I do think that Let Volir is a lot closer to how I imagined the post Great Schism Elites would go after Halo 3 if they became antagonists. No longer rooted in religion but driven by the pursuit of power and self interest.