Except that we have every reason to assume that First Order's training consisted of some brutal things, solely on the basis of what they were doing throughout the rest of the movie. Him being a janitor is mitigating circumstance in the whole story since janitors don't see a whole lot of action, but are we supposed to believe that he never, not even once heard tales from his comrades about what they were doing on missions? Or was the First Order a moderate imperially minded organization that all of a sudden became radical? You will notice that this was sarcasm and that there's no such thing as a "good" empire and that all one can hope for is that an empire is not too tyrannical, which we have no reason to believe on the account of what we've seen in the rest of the film.
I see what you're aiming at and I wholeheartedly agree that a character's true nature is bound to reveal itself at one point in his life in spite of unfavorable external conditions but if the light side of his personality was literally never nurtured, not even a tiny bit, then that part of his soul isn't gonna show itself much, if at all. Focusing on the hidden goodness instead of hidden evil is a nice contrast but you have to make necessary preparations for it to look and sound plausible.
This way, there's no escaping the impression that his defect is just another manifestation of "instant culture" that rules this generation, where no gratification comes gradually, everything is happening over night and without merit, whether it's love, skill, moral or other epiphanies.