The textbook scenario is that David 8 continues to be the villain. The twist would be, that this doesn't continue and he is an accidental hero, or deliberate hero.
Frankly, I'd prefer a diminished role for David 8 so he doesn't even get the chance to be a villain or hero, and I'm pretty sure that is what Shaw would have wanted by keeping him decapitated.
Unfortunately I might have to suck it up that this movie will make David 8 pivotal in his role in the movie, and also as either a villain or hero, and worst of all give him a closer connection to the alien/xeno/neo/spore pods.
Given that the trailers show a whole lot of killing going on, I see no reason why David and Walter get to be spared. Hopefully, xenos aren't too choosey and both androids get ripped to shreds the first time they encounter an alien.