His quick decision to join the brain proves that his only ambition was power. He wanted to be on the winning side. When he saw his goals as impossible to achieve because you no longer needed him, he sacrificed his own freedom for power again. That being said, they could have built it up better by making his true ambitions more transparent when you've been nothing but distrusting and hostile towards him from the beginning. They could 100% make his rant longer after you decide to free Orpheus and make his manipulations worse if you've been trusting towards him. I never trusted them from the start so it wasn't much of a shock when he said, "Aight, fuck this I'm out."
If it was just about power though, you would think that if you give him the stones then he would betray you and dominate the elder brain. He doesn't though. If you trust him, he does exactly what he says he'll do and there are zero consequences.
Ultimately, he wants to live. He only kills Ansur because Ansur tries to kill him, he wants to destroy the Elder Brain because it's a threat to him, he doesn't want to free Orpheus because he thinks Orpheus will kill him, and he joins the Elder Brain when he thinks he's no longer able to defeat it. I don't think that's a particularly evil motivation. He never lies to you, never threatens you, never charms you, never dominates you, and never betrays you. He's the only reason you're alive and he can kill you at any moment by rescinding his protection. He never even floats with as a possibility to you though.
It's unfortunate that siding with him shafts Lae'zel.
