Mean Spirited Sims receive a positive moodlet when making an enemy.
Mean Spirited Sims will have more probability of winning fights.
Mean Spirited Sims will autonomously use negative social interactions more often than other Sims.
Mean spirited Sims are able to troll on forums like evil, grumpy or inappropriate Sims. Then, they may wait for a response from the forum users. Not being responded on a forum will earn the Sim a negative moodlet.
A Mean Spirited Sim who also has the Good trait has most Mean interactions locked from them, limiting their cruelty factor.
Although many players believe that the Evil trait makes Sims the cruelest, Evil Sims can still have the Friendly trait, making the evil Sim still polite to other Sims (even without the Friendly trait, Evil Sims are still not particularly bad-mannered in conversation). Mean spirited Sims, however, enjoy hurting the feelings of other Sims, unless they have the good trait. However, Evil Sims like it when other Sims experience misfortune, which causes controversy among players about which trait makes the Sim cruelest. Combining the two (Mean Spirited and Evil) will create a spiteful sadist.
Mean spirited Sims actually get a mood bonus from making or being around enemies.
Mean Spirited Sims have a higher chance of winning fights unless they also have Loser trait.
The "Fight" option under Fight menu will be replaced with "Beat Up".