Question: how did your party deal with Nugrah? After what he did to his son I personally wouldn't blame them a bit if they just left him there to starve.
My players just fought him and then (I think if memory serves) let him go but that's largely because they never learned his actual back story. It wasn't something that he was likely to freely share and they had pretty thoroughly killed Staggy so he wasn't around to tell anyone the truth. None of the other bandits really knew either other than that for some reason Staggy kept him alive and occasionally had him heal the bandits (which they didn't like because he was creepy).
All in all I found it to be a pretty interesting back story that wasn't really likely to come out in play barring a fairly distinct set of circumstances. But yeah, if the party does learn of it and they do decide to execute him, I'm not really sure I'd consider it to be all that evil an act.
My party killed him in combat - but had I changed his spells a bit, levelled him slightly, played him a bit differently and made him a bit more of a challenge. There were six of them in the party :)
TBH He was never a great threat to them, although they didn't know that. He had the party pinned by the stairs, frightened to walk into the room and extremely irritated by swarms, monkeys and such like.