They sneak up and scare people, occasionally start a fight with a stranger or slap them, get a fiendishly delighted moodlet when a sim nearby has a low motive such as hunger or is tired, even if it's themselves.
Even evil babies get that moodlet, also they don't mind the dark so will never get the negative it's dark moodlet.
If there is a fire in the house my sims evil husband will often roll the wish to see her fiery ghost, also on the first day of a new job they will wish to see their bosses ghost, visit the graveyard is another common wish.
The will wish to give money to undermine charity as well.
I play with evil sims all the time and they aren't really that evil, more a comical evil than actually mean, traits like mean spirited, hot headed and diva are far worse but no sim will kill another sim it's just not a part of the game.