Norbert was a sociopath to begin with but kept up appearances as a hero and got hired to hunt monsters due to his skills. As the years go by, he slowly lets his ugly side out more and more until it was all anyone could see.
Serial Killer Alice knows that, eventually, she's going to be caught by Norbert. She kills his little sister in order to make Norbert more like herself as revenge.
Norbert is killing the monsters for the sake of becoming one, either physically or mentally.
Exploited: A Manipulative Bastard uses the fact that Norbert is turning into a villain to create discord within The Team which Norbert is part of.
Norbert vows to never become like his enemies, even if it means subjecting himself to a Fate Worse Than Death.
"It's war, and if you want to survive and win, you have to be just as brutal as your enemy."
Dicussed: "Well, what did you expect? Norbert was always the best at his job."
Conversed: "If you are planning revenge, don't. You'll become exactly like your enemies, like what happened in this show."
Every day, his foes show him new depths of horror, keeping him on the straight and narrow.
Norbert's goals are sympathetic enough that the heroes show reluctance in interfering with them; by doing so, they fear that they're helping the one he's after get a Karma Houdini, as the justice system won't give him the proper retribution.