I think there is definitely some kind of evil force that manifests itself in different forms. How can one justify mass murderers or sociopaths without saying that there is some element of evil there?
With that said, evil can be defined or explained in different ways, and it depends on perspective. As the above post stated, many times in depends on religious beliefs and whether someone believes in the possiblity of evil spirits or entities. If evil is defined as malicious, sinister, or ill-natured, I don't think anyone can deny its existence. It is just a matter of what form you think it takes (demons, devil, or just people).
Furthermore, I do not think good could exist without evil to balance it out, and I think in many cases evil yeilds good.Aat the risk of sounding like too much of an idealist or optimist, good always prevails. When tragedy or disaster occurs in a community, the community almost always comes together to find some good in the situation. As a general rule, people want to be good and want to do whatever it takes to make the good win out over evil. There always has to be some kind of balance, though. Can you imagine a world without evil? I don't see any scenario where that would actually work.