RE: How to Write a Sympathetic Villain
I absolutely love villains. If a villain is well written, chances are they'll be my favorite character. They're also the most fun to write! :)
The interesting thing about Deadpool is that he falls into both the anti-villain and anti-hero categories, since his allegiances shift and change so often during the course of his story.
Do rethink a few of the villains that you may not have thought too much about before. There are many of sympathetic or anti-villains out there, but they tend to get dismissed by most people. Mostly just because most people watch or read something with a "surface level" interpretation. Not that everything needs to be overanalyzed, of course, and sometimes a villain is just a villain, but it's a lot of fun to dig a little deeper. At least it is for me. :)