I might be evil
Does it have to do with where you went yesterday?
Yes, I went to the library. I've discovered few good books to read about philosophy, zoology and vampires.
It is good to reflect and realize that you have a problem. You can avoid behavior that makes you evil and become a better person.
What would I have to change?
From what I've heard so far, you probably don't need to change much.  It seems like you just found an interesting historical topic and read about it.  However, if you start thinking that you should actually do harm to others, for any reason, then you should find a way to resist those desires.