I let him live my first time through, but several hours later, I decided that was terribly out of character for Beast, the guy tortured and killed a LOT of Dwarves AND helped Queen Justinia advance her evil plot.
So I reloaded and this time I got him to teach me source, and then kept him leaving, interrogated him for information about his boss, which ended up killing him without me technically breaking my promise. I'm not replaying several hours I already cleared, but I think it's worth it.
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As for not having him kill teammates, I left my mages on the deck near the trapdoor to launch artillery strikes. That helped a lot. My second fight went a lot better than my first fight (0 deaths vs 3 deaths).
