I think it's really hard to have a fully bad character in this game. Like mentioned it takes a lot of work to have them behave in a non nice way. I try to use all traits in my game and the ones that people often consider bad are just extra flavour to me. Evil sim will occasionally do that laughter, mean sim is more likely to say mean things, non committal sim get tense sometimes. Gluttons I think are really adorable. I wish sims more consistently did bad interactions on their own but even if they don't the trait can still work as an incentive to play them in a different manner. I had a sim who was mean, hot-headed and insane. Very fun character to play the police career with. I suggest that instead of reforming them just leave their traits as it is and start playing as if they're trying to change their habits and deny their personality (if that's what you would rather do). It will likely work for the most part with traits kicking in on some occasions.
