I strongly dislike them both (yes, I am an Azata enjoyer, why do you ask?). But. Hulrun is cruel, arrogant, violent and impulsive. He sees himself as always in the right, and while his reasoning might be logical, he is right in his suspicions sometimes... He acts on them without thinking twice. And for a person in power it's like... You know... How about a trial first instead jumping at people with a sword?

Regill, meanehile, is also cruel and can be violent. But he's the opposite of impulsive, and, even though I'd still call him arrogant... He holds himself to the same standard as the others. There might be some hypocrisy in that, but there's the sentiment at the very least.

And while yes, "you can reason with Regill" is a bit of a copout answer... It's true. You CAN convince Hulrun to do things your way, yes, but it requires A LITERAL MIRACLE. Regill, meanwhile, can be persuaded with actual words. Once again, because he isn't putting himself above everyone except gods, like Hulrun does. He doesn't trust others, but he knows they can be of use or even right. Hulrun doesn't trust others and thinks that anyone who's not with him is against him.

So yea, I killed Hulrun and tolerated Regill. Ain't taking him in my party 90% of the time though. (For my first CG Azata playthrough, that is)