2 major mistakes blew this films credibility for me

Rating: 5/10 (4 June 2017)

This is a notable spoiler so stop reading if you've not seen it.
At the end of the film Walter and David fight. The outcome of that battle is never clear due to cameras cutting to another scene before the fight is over.
Walter hesitates on leaving the Engineers Fortress, it's not clear if it's Walter or David at this time due to a conversation moments earlier towards the end of the battle with Walter and David about which side Walter is on. We assume Walter has won initially when he rushes to the aid of the 2 injured individuals trying to get back to the ship. It soon become apparent in the medical bay that something is amiss when Walter tries to cover the missing hand he lost in a an earlier fight suggesting the wounds are not identical and this was self inflicted to look like Davids. This is the clearest early evidence Walter is in fact David.
Daniels (The primary female actress played by Katherine Waterstone) and David fight. This fight occurs before the final Walter and David fight mentioned above. During that fight, Daniels stabs David under the chin with her metal necklace pin. David doesn't have the ability to self heal, he needs stitches later. How did this wound heal? How does Daniels not notice this when she actively stitches him up in the medical bay right at the end? Why does David not change the trajectory of the ship back to earth if he is so intent on destroying the human race? Why does he continue with the original mission at the end to the new planet?
The effects were better in Prometheus, as was the costume design. The mass poisoning of the Engineers was a bit too easy, it was a scene that had a huge amount of work only to last a few seconds. I enjoyed the dialogue between David and Walter and the opening scene with Weylan.