Evil in a pure form exists only as a concept but in reality for those who believe in this concept (good versus evil in a religious sense...ie the Evil One against Jesus Christ). Other religious people may also believe in this concept.
This doesn't mean its an objective fact per se. An atheist for example would deny the existence of pure evil (as in the guise of an actual demonic being) but they might use the word as an everyday term denoting a monstrous act.
A Christian might believe in the physical reality of a demonic being and seek the help of a priest or similar if they perceive evil close by?
Since the introduction of psychology in the western world and subsequent studies into the workings of the human psyche it could be said that evil is nothing more than a manifestation of a disturbed mind, which is capable of committing atrocious acts. For the truly disturbed I'm not sure they're consciously choosing to act against the 'good', although history shows that many perhaps do decide to perpetrate acts of horror out of twisted belief and ideologies.
I prefer to think of evil as a psychological disturbance of some kind and don't go for the religious associations.