You only get to see his face if you let Lilith finish him off.
when i killed him i saw a robot face underneath and i'm sure Lilith mentioned something too, but i didn't kill him quickly, i waited ages, i wanted to hear what Jack had to say first..Lilith said ``hurry up and kill him or let me do it``
this was Salvador... Zero killed him instantly
maybe the game has different endings, because there is something very odd going on anyway... i.e ``remember when you killed him last time, that was funny`` plus ``oh no not again``.