That's a good question. I don't necessarily think doing an evil act makes a person evil. There are people who do evil things and feel a great amount of remorse over them. This doesn't necessarily stop them from doing it again, though. Sometimes the need to do these things isn't something they can control, which is what prison should be for. Protecting the rest of society from someone like this, not inflicting pain or punishment on the person.
When it comes to sexuality, I believe that if someone is turned on by kids, rape, etc. it isn't their fault. Acting on it is a different story, of course, but as someone with a somewhat unique sexual fetish (I'd rather not get into it) myself, I've learned that whatever forms our sexuality becomes pretty solid at a young age. My sexuality doesn't involve harming anyone, but it's weird enough that it has made being happy with a partner difficult at times, and I went through years of my life desperately trying to "rewire" my brain so I could be more "normal." It's impossible... This thing I have, it will always be the one and only thing that truly turns me on. There is nothing I can do about it. I don't think it's a learned behavior either. I honestly don't know where the hell it came from, but I can't control it. All I can control is how/when I act on it, and it's taken a lot of self-work to get to a point where I can accept this about myself... so I feel for people with "dark desires" in whatever way. It must really, really suck.
So, if someone has wicked desires and doesn't/can't control them, does that make them evil? Not necessarily ... Just my opinion, but my definition of true evil would be a combination of these things...
The desire to cause pain or harm to others, either as a direct desire or the lack of care if they are harmed in the process of carrying out another desire--for example, an arsonist who simply wants to light the fire and doesn't care who gets hurt, or a psycho who wants to perform live surgeries out of fascination.
No attempt to control these desires or the inability to control them. Or, attempting to control them only for selfish purposes (not wanting prison, etc)
Lack of remorse or feelings that causing the harm to others was "wrong" in any way.
I don't know enough about psychopaths to fully understand why they are how they are. I imagine it's a combination of a number of different factors, but I also wonder if people can just be born that way. No matter what the cause, I believe we should show compassion for all people regardless of who they are or what they've done. As a society, I mean. I wouldn't blame any individual for beating or killing someone who was hurting someone they loved.