Explained in:
1. showing a drained crew member during the initial attack (Bones tries to investigate, but has to move) (Bones)
2. Scotty being threatened by other aliens banding together who must have come from other ships (Scotty)
3. the actual life energy drain where we see him kill two crew members, with Uhura watching in horror (Uhura)
4. Jaylah explaining why escaping the camp was hard and going back not an option: "he has some kind of energy transfer, it changes him physically"
5. Jaylah saying that she doesn't want to end like 'all the others', once again reinforcing this has been ongoing for a longer time with many aliens (Jaylah)
6. the logs of the USS Franklin, where Krall tells us there is some kind of abandoned advanced alien tech on the planet, allowing for 'some type of energy transfer to extend life' (Kirk and Chekov)
7. The USS Franklin having disappeared 300 years ago, as mentioned by Scotty when he first finds out what jaylah's house is. The logs then complete the total image to: crazed soldier using alien life energy transfer tech to extend his life that also transfers physical qualities to the user from the victims.
I don't know how so many of you have missed all that since it's very much presented right in centre frame, but oh well.
oh, and:
8. When the surviving crew fly the USS Franklin to intercept Krall, Krall looks at the ship and says: "Hello old friend".
(which I'm pretty sure was also a Nimoy line in one the previous movies - TOS that is)
The one actual inconsistency with this plot is why Krall didn't send regular patrols to check his old ship, since holo-cloaking is all fun and all, but he and the two others would / should remember it's there. Maybe they were changed so much that their memories also became scrambled, but that's really the only big fridge logic point, as far as I can tell.
Also, the alien miners who left the technology on the planet were likely all killed by the unleashed bio-weapon and managed to split it and shoot it into space at the last minute but too late for them to survive.