But at the same time, the commonplace statement about them is true: every character is the hero of his own story. Each has a justification for his actions that is convincing to him. It's fun to give these people voices.
You have very accurately described the difficulty of presenting my books on film: many of my characters are alone most of the time, and when they do talk, what they say is mostly lies. That can make for a pretty confusing film.
Reading a novel in which all characters illustrate patience, hard work, chastity, and delayed gratification could be a pretty dull experience.
If I don't have a project going, I sit down and begin to write something - a character sketch, a monologue, a description of some sight, or even just a list of ideas.
Once you have invented a character with three dimensions and a voice, you begin to realize that some of the things you'd like him to do to further your plot are things that such a person wouldn't, or couldn't, do.
What I look for in any character, good or bad, is whether I can hear him speak. If I can imagine him that clearly, then I can write about him.
I do like to explore evil characters in my books.
The characters you refer to as predatory and unsavory are useful. They're the ones who make a novel into a thriller. They're active, and most of the common virtues, the signs of a good person, are not.