Spike is interesting to me, and it's because he exists in the gray area between good and evil, noble and ignoble, etc. He gets me asking questions. A thread like this would never happen for Oz or Tara, and IMO there is far less to gain from trying to understand them because they are much more straightforward and good. They are not challenging to me in the sense that they don't inspire any reevaluation of myself or people in general.
Nietzsche- "In heaven, all the interesting people are missing."
You can learn a lot about the way people are from instructive examples like Spike, but IMO a thread like this is best used afterwards when your attitude and opinion of Spike is turned back towards yourself. I think that is the final point of interesting characters, if the point is to learn something from the experience.
I like Spike. I don't like Spike. There are reasons to feel both ways. What does that say about myself? What does that say about my own personal code of conduct or ethical positions? Where do I (honestly) stand in relation to what this other person or character is, do I exhibit some of the same qualities in disguised form? Is my dislike of that character hypocritical in some way difficult to understand? If that is the case, is my dislike of the character a form of blindness about myself, or even a form of self-loathing? These are possible examples of productive questions to ask yourself, especially if you have a strong reaction about someone or some character.
Most people decide whether they like someone or not and stop there. They fail to ask themselves the hard introspective questions afterwards and miss a chance to learn more about themselves or the way people work.