Then again, that Marcus says Khan is X can probably be taken as solid evidence that Khan is not X...
The only reason Marcus even utters that phrase about war crimes is because he is attempting to drive a wedge between Kirk and Khan, in a situation where he knows Khan is not present to contest the claim. (Although being a devious man himself, he might have done well to suspect that Kirk would
be piping the discussion to Khan's cell...)
Khan's blood might be something he did to himself in order to play out the initial moves in the game against Marcus. If he can use his intellect to design starships with technology completely alien to him, he can probably do wonders with biochemistry, too.