I would have been so much happier with the film if John Harrison had just been a kick-ass S31 operative with access to loads of classified technology, rather than what they turned him into (I actually groaned in displeasure in the cinema when the reveal was made).
I totally agree. Because when the big moment came I think the film started going off the rails for me. It was like the writers just couldn't contain the inner fanboys inside them and just had to go there. Perhaps not because they thought they could do it better but maybe just to put their own spin on it or to write a character they probably grew up wanting to play with.
But that was totally unnecessary really for the story they were telling. Harrison was compelling enough. His motivation didn't even have to change. And if Harrison had been left alone I don't think the film would've veered (maybe) into a weak imitation of superior works.