It seems like I spend so much time thinking about who the good guys are when I write my fiction that I sometimes forget that bad guy needs love too, so to speak. Since plot lines revolve around conflict there has to be a source behind that conflict, and that would be the antagonist, more commonly known as the villain.
There are different approaches to creating a good villain. One way is to have him or her truly evil and completely irredeemable, sort of like a Count Dracula. Your readers will hate him and root for the good guys to wipe him out. That would be a the sort of villain you can kill off at the end of the story, and your readers will leave feeling relieved and satisfied.
A more complex, and interesting, approach would be to create a "conflicted" villain. Instead of a purely evil Count Dracula, you could create a villain more like Barnabas Collins, the reluctant vampire of Dark Shadows fame. Barnabas had been a good guy until the vampire curse was placed upon him. Once that happened, he was left despising what he had become. He both hates, and deeply regrets, all the things he's had to do along the way in order to survive. Thus he becomes a "hero/villain;" an antagonist whom the audience could root for, because they too wanted to see him cured of his affliction and end up with the girl, be it Maggie, Vicki, or his true love, Josette. In the interim, they sure get to see him wreak a lot of havoc.
Some storytellers like to take chances by having their protagonist, or hero, go bad. That can be a tricky approach. If you're going to attempt it you need to have a character with plenty of redeeming qualities, otherwise your readers won't be able to make a connection and they will not root for him or her. By the end of the story he or she will have to renounce all the bad things they had done earlier, and as well as be willing to do whatever has to done to make up for the sins of the past. If not, your readers will not be satisfied with the ending, assuming they stayed with your story until the end.
Anther way to conclude your story would be to end a tragedy with tragedy, especially if your hero/villain has done some really nasty things in the past that he or she really can't walk away from. At the very end of the Star Wars saga Darth Vader renounces the emperor and turns away from the dark side of the force, but in so doing he has to sacrifice his own life in order to save Luke. It certainly made for a dramatic, and satisfying, end of the conflict.
So there you have it. With a little imagination, and a few character quirks, you can create develop interesting and memorable villains who can keep your readers engaged. And that's what good storytelling is all about.