People who say they don't believe in evil, I don't believe are stating their beliefs accurately.
If someone breaks someone's house, kidnaps and rapes a child. Tortures and kills it... to the Hindu, I ask, is that evil? What is it, if not evil? (I believe Hindus differ in beliefs, because i spoke to a Hindu, who believes that's evil)
However, I think they would be more accurate in saying, evil is determined by the person. So for example, one person may view a particular act as evil, but another may not. Somewhat like people today view morals.
So they do not believe is an "absolute" or standard of evil.
That seems more of an accurate concept to me.
What is sin to the person who doesn't believe in sin? What is evil to the person that doesn't believe in evil?
The reason you have a hard time wrapping your head around this might not be their fault... Instead of just assuming what you think they are thinking, you could just ask them what they think.