It was necessary to have cut off his arm in order to save him but honestly I don’t think they would have had enough time to finish it anyway. This way he was able to save others while being spared the pain of having his arm halfway cut off.
Also, I have been wondering this whole time about the people in the walls. Do they maintain consciousness? I mean they become one with the wall right? If so, wouldn’t that be absolutely awful for Alan to have his last sense of touch be pure agonizing pain?
Oh! What is her name again? Carla? Anyway it’s the captain’s spy sister! Haven’t seen her for a minute. Something big must be going down.