How do you get "in touch" with evil -- if you don't possess it inside of you?
Good question. I think getting "in touch" with evil is admitting it is a basic human trait. When I write an evil character, I try to 'humanize' the character. This is someone's son, brother, husband or friend. Evil is defined by acts. Acts are taken based on motivation. Having motivations people can relate too, makes the 'evil' something they can relate too.
One of my books, 'Drop of Evil' was particularly hard to write in this fashion because it's based on the premise that evil is genetic. In fact, the whole reason I started writing the story was trying to look at evil motivations from a different perspective. However, the one overall theme persists...evil can be found in everyone (just as good can be found in everyone).