What happened to Walter?

He did a Covenant with David. They both share the Walter body.

@BlackGooDrinker. Totally plausible.

so now there are two Davids, interesting.

I think he just killed Walter, I don't really see why he would make another David to continue his experiments on the Engineer homeworld, where the experiments are considered to be what he calls failures, he has his successes aboard the Covenant.

We see David reaching for a knife so I think he stabbed Walter to a milky death off-screen.

its is something left open to debate thats for sure... But RS did confirm that David killed Walter and took his place and Cut his Hand Off to pass as Walter.

Is Walter 100% Dead? Who knows... Walter can repair himself but this maybe limited to Minor Repairs.

As for David.. we have to ask is he going to play a One Handed Android in the next movie?

And would a Colony Mission with the intention of setting up a New World and never returning to our Solar System, really take just ONE Synthetic without no Spare Parts?

So i think David will Re-build himself better.

@BigDave I would love to think that Walter will appear in something else ;)

What does it take to destroy and android? We saw that David was still viable even after His head was torn off by the Engineer on LV-223.    Walter seemed to have the upperhand in the fight against David.   He also had regeneration capabilities.   It's quite possible that David damaged Walter enough just so he could assume his place on the Covenant.   Walter lives!

Walter may live but i am not sure they would bring him back though....

A more Logical and Shoe-horned way would be that David uses Daniels for Face Huger Number 1....

David then manages to gain control of the ship to basically do as he commands, and then he finds some Walter Parts...

He Then Builds himself a Walter, and then Uploads his own Soul into Walter, with Technology on the ship...

David has a NEW Body... he has a Improved Body.

Walter Models are HOWEVER.... somewhat Synthetic Constructs like Elden from the Fire and Stone Comics.

So ultimately this Walter be it David creates to serve him, or David transfers his Soul to... can be infected by a Face Huger and this gives us the Bio-Mechanical Xenomorph

Unfortunately with most issues in the film the most interesting things happen off camera.  David either talked Walt into giving up, or David won and substantially wounded Walt.  Either way it's a poorly written script.  Let me ask the common folk, if you sent an AI on a trip to protect humans, what would you program him with?  Hand to hand, check.  Medical, check.  Engineering, check and so on.  Why not?  So with the fight, Walt was on top.  In that situation you don't listen to your opponent talk.  You ruin his (it's) eyes then break it's neck.

@cryofodder I'm with you! Walter was just incapacitated long enough for David to escape. David intimates as much in the novelisation, saying 'David' is not happy and will be on his way... Makes me wonder what info Alan Dean Foster had access to in order to write the book...

David made Walter stop punching his face when he said to him he should choose a side.

This was in reality just a clever distraction which gave David enough time to grab a knife before Walter continued punching him.

He probably stabbed him into a vital/critical sub-system, which may not be the head but another part of the android body which has a weak spot.

As we know the alien-humanoid-androids post/at Nostromo timeline are a threat even without a head attached.

All he would have needed to do was stab Walter in the neck again to incapacitate him. He didn't need to kill him to escape.

think walter would see that coming

Not necessarily... He doesn't quite have the ability to anticipate such unpredictability. Maybe he simply waited for Walter to hit him again, then moved at the same time...

hhhmmm nah

Love Walter. Breath of fresh air in a horror universe.

I'd keep him alive. As I suggested: his head is in a sack(severed by David) at the bottom of a well. How he escapes is a good story.

What happened to the Giant Engineers mother ship? 

I suppose he could have taken Walter's head off completely, judging by the fact that a Synthetic can survive such an attack.

I didn't consider the big docking ship... Hmmm... It must have a dock of its own somewhere...

David overpowered Walter and took over his body. he transported his soul to Walter and fled with him. and left his old body or duplicated that there are two David's and Walter's software has replaced his or simply turned it off. he had the same wounds on his face as Walter and a severed hand. he had to somehow take over his body and everyone on the ship thought it was Walter. took Walter's body because it was perfected. I wonder what about David who was on the planet 4. turned off or there will be two of them and they will create stories together.

Ridley Scott seemed to imply that David had killed Walter and taken his Clothes and then Cut his Hand Off...

This is how i saw it too....

I will add though that i expect that David would Transfer Himself into a Walter Body on the Covenant (surely they have Spare Parts) or at Least we see David would be able to attach a Walter Hand to his own Body.

It is likely that Walters Body is on Planet 4 and that THIS is the way the Engineers would Discover where the Covenant Ship is heading.

But we cant 100% Rule out that David is on Planet 4 and we have a Walter with Davids Soul on the Covenant, we then in effect have TWO Davids ;)