Nope, I killed him without doing anything crazy. I just ran around looting bodies and when I was about to take Lae'zels inventory and armor and depart I noticed he had 3 hp and hide/shot him real quick. He doesn't seem to drop anything.
I'm surprised he got to such low hp, did you have lae'zel attacking him while you looted or something? In all of my run throughs of the prologue so far, he beats the crap out of the mindflayer he's fighting, even had him kill it once before I triggered the cutscene.
