Yay, Shaw! Even if she doesn't have a proper appearance in the actual film at least we got this. Love the design of the Scorpionaut. It has that same great 'totally alien and asymmetrical' look to it as the Derelict/Juggernaut while still clearly being something new. Very nice.
And yes, David's "I learned their ways" was very ambiguous, in a good way. As has already been speculated here, maybe whatever he learns about them in that time is what ultimately makes him decide to wipe the Engineers out? Or maybe he still just holding a grudge about one of them decapitating him.
Either way, it was a cool switch from "compassionate David" to "about to coldly commit mass murder" at the end there.
Oh David, we just never can tell with you, can we?