Do you have parenthood installed? Toddler and up to Teen have an increased gain in character value up or down, once you autonomously use mean interaction, their conflict resolution, emotional control or empathy would decrease in an accelerating rate once it reaches enough low they will autonomously use negative interaction (including mean), though from what I observe it also affected Young adult and up but very slightly and you cant see their character value, I had my young adult sim who didnt have parenthood trait autonomously use mean interaction because I ordered him to few weeks before(sims time) now they do it without my supervision and he was a goody two shoes before.