Playing a LE campaign. Picked up Otis in Nulb as an NPC. Did steal his stash but he did not attack, I got the "I work for the King, etc" dialogue.

Everything is fine until we arrive at the outside of the Temple when he turns on the party. They kill him. Why did he attack? Just because they're evil? I can't find any walkthroughs or FAQs that have that scenario in them.

What did he say when he attacked? If he accused you of stealing his equipment, then that means there is probably a check at that point to see whether you did so.

If not, it's probably because he realized you're evil, and he just wanted you to get him to the temple.
Frankly, taking a good NPC into an evil party (or the opposite) is asking for him to turn on you.