"Do you personally have the capacity to do better?" Challenge accepted.

Rating: 1/10 (15 August 2017)

That movie is awful. I could write pages about it, but I chose instead to focus on one of the absurd arguments put forward by some reviewers.
I've read ALL the reviews (over 1,400 at the time of writing) and several tried to excuse the movie by asking people who disliked it if they could do a better job (ex: "Do you personally have the capacity to do better?"). Now, this is a ridiculous argument, because if you buy, say, a croissant, you expect it to be comestible, right? If it's burned to a coal-like lump, will you accept the "can you do better?" argument? I don't think so! You paid, you expect at least an acceptable product.
Still, I think that the script was ATROCIOUS and yes, despite not being a professional scriptwriter, I could have done better. Rewriting the whole story would be pointless, so I'll just present how a much better ending (with a real twist) could have easily been done. My proposal is not perfect because I chose to minimize the changes and to do a good job the whole story would have to be scrapped. It still has a few plot holes & excessive suspension of disbelief.
For my ending to work, there needs to be a few changes in 2 scenes.
1) The fight between David & Walter

We must see clearly Walter's face being cut in several places, then healing, during the fight (to establish that he has some auto-healing abilities)

The fight must end with Walter losing: we must clearly see him disabled, maybe after the stab in his neck.

2) The stapling scene
Back on board, "Walter" is stapling some cuts on his face. Daniels joins him and helps. We must have a closeup of the robot head that shows on the back of his neck, just below his skull, a wide (2 inches) cut, badly stitched, and a visible "lump", just below the cut, protruding at least half an inch (showing that there is something under his skin).
To the viewer, this scene should suggest that the robot is in fact David (because of the cuts not healing).
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My ending: it takes place just as Daniels is about to enter her sleeping pod. The camera shows that her gun is behind the pod.
===
"Walter": You're next Captain. (she is about to enter the pod)
Daniels: Thanks Walter. Can you give me my towel, please? It's just on the rack behind you.
"Walter" turns, takes a couple of steps toward the door. The camera shows him from the back, we don't see Daniels.
Daniels: By the way, _DAVID_...
"David" (half turning his head back, with an interrogating look on his face): I beg your pard...
BOOM!, BOOM! (violent gunshots, the robot is ripped apart, limbs flaying & fake blood sprayed everywhere)
The camera turns and we see that Daniels is repeatedly shooting him in the back.
Daniels (screaming while shooting): F-ck you! (BOOM!) F-ck you! (BOOM!) F-CK YOOOOOU! (BOOM!)
click, click, click (empty gun noises)
We see the robot in pieces on the floor, a real mess. Some limbs still twitch a bit.
Daniels drops the gun and slumps on her pod, exhausted.
Daniels: all dead... All those deaths, just because of ONE crazy robot...
Tears come out of her eyes. This is a very emotional moment, grief mixed with exhaustion have taken a huge toll. This scene lasts at least one minute. Then, slowly, with a blank look on her face (too much grief), we watch her slowly preparing for sleep then enter her pod. One last look at the remains of the robot on the floor and she closes the pod. She shuts her eyes, waiting for sleep to come...
Ten seconds pass. The camera shows the pod from outside, when an OMINOUS human-like shadow looms over her pod.
A few dramatic seconds pass. Then we see Daniels (realizing that the light has suddenly changed) reopen her eyes.
Half a second passes and she screams "NNOOOOOOOOOOOOOOOOO!". The camera turns around and we see _DAVID_ -with a half smile-, holding _WALTER_'s severed head in his left hand, waving with his right hand at Daniels.
David: You did not disappoint me Daniels. You see, as soon as I saw Walter, I imagined replacing him. But when I realized that Walter could heal his wounds, I had to change my plans. I knew one of you would probably notice that the cuts on my face were not healing... Plus I did not want to lose a hand. So after I had disabled that poor Walter, I added an additional circuit of my own design in his neck. You see, our brains are too complex to be directly modified without special equipment, but it is still possible to access the input/output stimuli processor. That's a flaw in our design that I had long been aware of. This allowed me to modify Walter's perception and make him "forget" about me then send him back to you. I also managed to disable his healing unit. I was curious to see how much time it'd take you to notice...
David is walking around the pod, gloating. Daniels frantically tries to open the pod, but they only open from the outside.
David: Think about it, if it had been ME, would I have helped you kill my creations? Anyway, now I have plenty of materials to experiment with. And, thanks to you, I will not have to kill Walter to protect myself from his possible interference. Thanks again!
A smile. A last wave of the hand, and David walks away.
We see Daniels with a distorted face, screaming, falling asleep as the pod puts her in hibernation, for a LONG, VERY LONG sleep full of nightmares.
THE END.