Pokémon Sun and Moon - Okay, So...

Actually it's not quite so simple. Nihilego doesn't add any aggressiveness to it's host, it only adds powers and removes inhibitions. essentially anything associated with, well, ego. your relations and perceptions in the eyes of others that affect normal people everyday. if you've ever thought "I want to punch that man but I'll look like a psycho" or "I want to steal this but I could get in trouble with the law" then Nihilego will remove those "but"s. Essentially it takes that fear from you and makes your decision making more along the lines of "I want, and so I DO!". if someone has suppressed violent tendencies it will let them all out and remove suppression… however if they're already let out (because they don't care what others think) then it won't push them any further. in theory it doesn't even have to be a bad thing if the person acts not on the ego of it's namesake but super-ego; If you do good things not for the sake of others but because of your own personal morality, you will continue to do so. It's a controversial term due to history, but if you know the original meanings of ubermensch and live by them you'd be largely immune to any mental change, since your impulses are internally controlled.