Why? [Spoilers]

AI has shown to gain some trust again in Aliens, Alien 3 and Resurrection. 

Exactly. They should have marked one of the androids, like you do with pencils at school! From the very first moment I heard Michael Fassbender was cast in a dual role I knew it was a sign of a rotten plot device. And the sequel already has a 10 page treatment and I can tell you from know that "Awakening" involves Daniels waking up with no recollection of what happened in Alien Covenant. I call that quality writing.

I hope we don't create a very advanced AI like this in the future. It would be dumb of us to do.

If David was really on the ball, he'd recreate himself, effectively cloning himself and then go forward to building a Synthetic 'species' or civilization.
He obviously considers himself to be 'Perfect' so there's that angle.

I am not one to usually rewrite scripts, but that David-walter switcharoo, to me, could easily have been remedied if....

Instead of introducing himself, David watched the crew first, and spotted Walter (honestly, he probably was, anyway). Before he introduced himself to them, cut his hair, and mimic Walters identify BEFORE meeting them. At that point, the crew would surely accept they are just naturally identical. That way, we, as an audience, are not asked to believe nobody was suspicious at David suddenly copying his brothers appearance completely. 

I guess in the next movie, we get to meet the human David; a la Bishop in Alien 3.

Colonial Soldier I know there must be a joke in there about Div'Ed fingering the Bishop.

Xenocentric

asked to believe nobody was suspicious at David suddenly copying his brothers appearance completely. 

I think it's less damning than some are making it out to be. 

The crew had no concept at all that one should ever be suspicious of an android.

By the time David's deeds are revealed, no one actually has the time to sit down and process the thought "hmm we now know that something is wrong with this android. I wonder if there's some motive to cutting his hair".

Everything at that point was happening so fast, I couldn't even imagine having time to think at all

Why did the old David turn back on humans?

Because he regards humans as being entirely unworthy of their creator (he says this explicitly about Weyland). He ridicules the very notion of a colony ship because he says humans are a dying species. In his perverted vision of the future, he sees it as his duty to stop humans in their tracks. Is this utter madness or just a question of morality? For me, matters of morality, slavery and evil are things that are great about this movie. Far more expansive than just a few monsters running around.

As to why he wishes to see the end of the Engineers, he must also regard them as being unworthy of their creator (whoever that happens to be).

Why did the old David experiment [on] Shaw?

In order to create the 'perfect organism'.

Is the point of movie is to never trust AI? I think this is the moral of the story here.....the AI was flawed and it turned back on everyone.

No. If anything, the point is that free will itself can be fickle. This is, of course, an old trick of the Bible: sin is a byproduct of corrupted free will.

>This is, of course, an old trick of the Bible: sin is a byproduct of corrupted free will.

Interesting, is there a scripture for this? What's the scripture?

The disobedience of Adam & Eve in the Garden of Eden (which can only arise through free will) and subsequent blight of mankind is known as Original Sin.