At the end of the day, most stories - especially in paranormal YA - boil down to that eternal struggle. Good versus Evil.
In my opinion, it's easier to write the Good. After all, we all like to think that if we were in an epic battle between Good and Evil, we'd all be on the side of Good. Right? And since we write what we know, and we write from the heart, writing the Good is easy.
And writing the Evil is...fun.
Well, it's fun, but it's hard. It's really easy to slap a nameless evil entity onto the page and have it wreak all sorts of havoc. But it's hard to give that evil a name, a backstory, a motivation, a purpose...a soul.
The very best villains have all of those things.
Take a look at one of the best villains in recent years, one that's become a household name: Lord Voldemort. He's as evil as they come. And at first you think that he's evil just to be evil. But little by little, we learn things about Voldemort's past. He was an orphan, just like Harry. Hogwarts was his only home...just like Harry. Rowling weaves a backstory for Voldemort that is so rich and so emotional that we start to relate to him. We start to see that, given a different set of circumstances, Voldemort could have turned out like Harry - good, honorable, brave. And likewise, given a different set of circumstances, Harry could have gone down the same dark path as Voldemort. Some of the best villains are twisted mirror images of heroes. As the series goes on, Voldemort's story gets more and more specific, and he becomes a better and better villain.
This was something I had to learn in my own writing. In the original version of SHIFT, one of my major antagonists was a fire-and-brimstone, holy-roller preacher. Then it went to the editorial board at HarperTeen and they asked me to revise. They wanted me to take out all the religion.
When I got that note, I have to say I wasn't surprised. When I was writing SHIFT, in the back of my head I always had the thought that someone was going to get offended and make me change it. And I wasn't too attached to the religious aspect of it - that wasn't a focal point of the story. So I started to think about what to replace it with, and I came up with a shadowy, nefarious corporation called the Guild.
My preacher turned into a slick corporate honcho, with a weaselly assistant trailing him everywhere. Neither of them are as they seem. As I started to flesh out these characters, and weave the Guild into the mythology of my story, the good versus evil arc really started to take shape. Now my main character had much more powerful enemy than just a small-town preacher. She had an entire corporation to take on. After all, the most dangerous thing a villain can have is money. And who doesn't love to hate an evil corporation?
By forcing me to take out the nebulous religious aspect, my editor gifted me the opportunity to create a more specific, dangerous, far-reaching, and scary villain.
It was a lot of hard work - it was a complete and total overhaul of the story! - but the story is so much better for it. And now when I sit down to write those villainous scenes, it's fun. Because after all, although we know we would fight for the Good side, it's also fun to be a little Bad sometimes, too.