Those supposed to be In The Know say that all the characters a writer creates retain some facet of the writer's character.
I don't know if I agree with this altogether, since most of my characters are amalgams of people I know, or have known. I explain my process of character-building here for those interested.
But for my primary antagonist (Agakari; think Hannibal Lector on steroids and less inhibited), I said that I dug him up from the deepest, darkest parts of my own psyche. This shocked Madame Editor considerably when I told her, since she only knows me now in this angelic manifestation.
But the High Inquisitor is not me. He is, however, someone I might have become (supernatural powers excepted, of course...) had I taken a different path in life. This is true also of my protagonist. Jonas is not me, but I could have become very like him had circumstances been different.
When writing a character, however, the writer must somehow enter into that character's 'mind', must think, must feel, must react, like that character. At least this is how it is for me; if other writers have an alternative process, they can explain it to you in their own blogs. Perhaps this is what Those In The Know mean by some part of the writer being found in the character.
It did, however, present me with a problem. I didn't want to enter the mind of Agakari. I didn't want to mix the bloodlines of men and beasts with magic darker than the Styx, nor Transform men into soulless Gargoyles, nor produce progeny from my nearest living female relative (Warning: the previous sentence contains spoilers for those who may read my novel...).
And so I intended to write my antagonist in a similar way to that employed by Tolkien with Sauron; as a presence that permeates the book, whom one never meets, but is always aware of. As time went on, I realized this could not work, since Jonas had to meet Agakari several times in the book, as did one or two other characters. I had to write from within the antagonist's mind therefore.
So at last we come to the point of this post. I have mentioned Observation, Imagination, and downright Theft as part of a writer's necessary skills, but to this point I have not made much of Introspection.
A writer needs to know what he or she is capable of.
I have very little time with those who believe humanity to be essentially good: all the empirical evidence seems to me against them, apart from the Scriptures I believe in. But we don't, of course, like to think of ourselves as inherently evil, as containing within our own psyche the seeds of our own destruction. It isn't pleasant, it lowers our precious self-esteem, which, of course, is unthinkable in the modern Zeitgeist.
Perhaps it will help to ask you, gentle reader, to remember the last time you wished a fellow human being harm. The Democrats among you need only think of the sitting President, and Republicans of the previous. There, see how easy it is?
Now for the hard part.
Look deep into your heart, and be honest with yourself (be careful; your own heart will deceive you into thinking you are really quite a nice person), and find out what you would really do to someone you truly dislike, if you could get away with it.
Did you scare yourself? If you didn't, you weren't honest enough.
Now take what you found, and give it to your antagonist, and you will have a bloody good villain.