Do you believe in evil?

What is the importance of evil in plot? Do you believe in evil? Melly from All Kinds of Writing discusses:

I do envy those who can imagine evil, pure evil, in any form - physical, mental, metaphysical etc. They have more options. I now know why I tend to stick with science-fiction (oh, I can certainly imagine scientific things, or future societies) and mainstream (I can also imagine different, quirky, "evil" human behaviour).

At the same time, I have a hard time with horror/fantasy/religious concepts in my writing because of my very realistic, pragmatic world view.

I believe in evil, but I also believe the definition of evil differs from individual to individual.