Palamedes says "he condemned me to wander the Shadowrealms as
punishment for an old, old crime."
He also didn't seek anything from Palamedes, Will and Saint-Germain in exchange for his services. I know they threatened him but he would have been justified in asking for something.
He cares about the planet too, which is important. But I agree with you now, he isn't a good guy, I read a passage wrong in the Necromancer. When Palamedes says "You could stay and fight, you did that before." I thought that meant he fought for Marethyu, I failed to read the next line "And we lost"