I think the Master Chief is no longer human (as we understand it). I am not sure if anyone mentioned this before...
1. The Librarian biologically modified the Master Chief.
2. The composer was used to burn up all the humans at the station.
3. It looked like the Master Chief was killed...
I think the Composer burned up all of "the human" in him.
Those that believe the unmasked face is normal human are missing an important cue, the music cuts off and a sound effect plays to enhance the viewers gasp.
