I've never had too many problems with the Evil trait really; one of my first sims ended up being given that trait and I've learned to morph his unique personality around it. He has just the right amount of mean sometimes but is not overtly mean when I make it so. It's Mean sims I generally dislike, but it's one of the few negative traits that have any real influence in anything. The Mean trait can carry itself to various interesting stories, like ones involving abuse, but it is still possible to have a decent relationship with a Mean sim (i.e. one of my sims had a boyfriend who was occasionally mean to him whenever they went out, and ironically it was my sim that always had to apologise to him after he casually insults him; abuse much?)
If you personally find it unreasonable for you, I suggest simply saving you the trouble and apply the positive character values onto the Mean/Evil sims if you have Parenthood, especially the Good Manners one. Applying that on my fave evil sim actually helped me round out his personality and now he doesn't occasionally insult his own mother, wife, and toddler son anymore and only relishes in people's misery now.