It's not that writing an evil character can "make" or turn you evil, but rather that you can understand evil perspectives and so draw from that in your writing. You were evil to begin with, and that's okay. [link]
In the end, (in my opinion) evil is about what you do or don't do, not about how you think. If you're getting into a character enough to affect how you perceive your own reality, it's time to take a step back before you start acting on that, but stepping into character doesn't make you evil.
Let's make a metaphor: Suppose you have a guy who comes from a family of alcoholics. He himself doesn't drink (I guess he was smart enough to realize alcoholism is genetic so he'd probably end up going the same way) but he thinks about it an awful lot, especially when it's dark and cold and he's alone. He's thinking like an alcoholic - he even has the potential to be an alcoholic - but he isn't one because he doesn't act on his thoughts. Same goes for any kind of thoughts, good or bad.
That's like asking a Christian if hanging around their Buddhist friends turns them Buddhist. We can all explain what we believe as right and wrong, so even if we study something we believe is "wrong" it won't alter our views. We understand this is not part of our belief system and move on.
People are rarely classified as "good" and "evil" in real life, which is the other problem I have with this question. Even murderers can show kindness and Christians flip the bird in traffic jams. "Evil" people don't actually exist- they're doing something they believe is good or worth doing, even if it is for revenge. They really believe in it, so how can they think of themselves as evil? I feel that writers who make characters by stereotype are poor writers altogether. Creating a good guy/bad guy is not the most creative or exciting character cast out there.
The point I'm trying to get at is this: I don't consider any of my characters evil. They all have a motive they believe in, and what they're doing makes sense to them. Just because you understand that motive and spent time developing it doesn't make you good, evil, or anything else. It only makes you well researched, and good at developing characters.
i dissagree that to be evil you have to act. you may be evil but not act because you are affraid or otherwise incapable of acting. for instance: can someone who is paralized not be evil? sure he can.
so, does it make you evil if you write about evil characters (serial killers for example) and you really get into the psyche of the character? no. you are evil if you IDENTIFY with said character. if you write a horror story and think to yourself, this murderer/rapist/theif... etc. is so much like ME! then there is a big problem.
lets also remember another thing. some people are conflicted. they have evilness in them but they fight it in various ways. one of these ways is writing about it which syphons some of the pent up emotion/anger/desire/whatever. it's a release that stops them from acting at least temporarily. so are these people evil?
one more thing is this: does it turn you evil if you write about evil? i think any exposure and especially prolonged exposure to any evilness takes its toll. none of us are 100% resiliant to it. kinda like the ring in lord of the rings. stay with it too long and it affects you. if you watch violence you will have a more violent personality. it doesn't mean you'll go around beating people up but it will have its effects. for instance. i live where terrorism is usual business. in a place where there is such a term as "emergency routine". i see how it affects those around me and i feel its effects on me. i try to fight it but there is no complete deffense.
evil is someone who has a strong desire to inflict pain on others, ruin, destroy, break, mutilate and so on and finds pleasure in other's pain and suffering and of them getting hurt.
this is not only with regard to humans either. one who enjoys seeing animals suffer like the kid who burns the antlers off an ant with a magnifying glass and enjoys watching it suffer is as evil as one who enjoys killing people.
I see your point, but I disagree with the example of the kid and the ant. Speaking as someone who used to torture ants, actions of a child do not necessarily transfer to adulthood. These days killing an ant makes me queasy. As an adult, I understand the responsibility of having power over such small creatures. Back then I didn't know any better. So while in many cases animal torture will precede human torture, it is not so black and white.