Writing is the ultimate form of expression. You can write about anything. The stifling rules of reality need not apply, for the only limits are those of your imagination.
It is an incredible release. All those pent-up frustrations can be loosed upon the page. You can be yourself for a change, rather than acting the part one is forced to play in real life.
Evil in literature serves a purpose: usually to be overcome. So, the destruction it wreaks in the real world is contained in the fictional one. The wickedness which vexes our daily existence can be dealt with entirely and to satisfaction.
One can write realistically, without poisoning the work with the failings of the real world. We have a sense about how things should be. In fiction, this state can be achieved.
Characters tend to be flawed, just like real people. But in a story, these things are meant to be recognized and struggled against. If they remain, somehow it is to prove a point or they are to prove fatal for the villain.
No art form has more freedom of expression than writing. Dull stories are born of dull minds. And thus, they will attract few readers. But to really live, one need only pour themselves onto the page.