You pretty much have to be fair! There's a moment that leaves room for interpretation. Krall see's his reflection in a shard of glass, pauses then grabs it. It could be seen as him thinking as he see's it, then acting on the impulse to stab Kirk - but to me it looked more like seeing his real self for the first time and having a brief moment of contemplation.... only for that reminder of who he was anger him and resume the fight. Krall's arc was fine as is n my view; not every bad guy can, or should be, redeemed. The down side was that they didn't hammer home the story of why he turned, but it would have been a bit much to throw in at that stage of the film, and having too much background early on would kill that eventual reveal. So I was fine with it not getting too much depth.