There are an infinite number of master chiefs. This is known because when you die in game, a new one comes. I think that one of the newer master chiefs was evil (the same one we see in the locke trailer). Locke kills him and then a NEW master chief comes and pins down lock as such in the master chief trailer. This can be seen in the fact that the scene is all around worse (cracked chief statue) when the NEXT NEWER master chief comes after Locke kills the evil master chief.
All i’m saying people is stop trying to spoil the game for yourself. Let it surprise you.