In my ramblings on the web I also found the book was based on the early failed revolutionary Dechaev.
The story reminded me very much of private therapy sessions in which the doer evades responsiblity for actions by reporting the emotional feelings as a way of getting rid of guilt.
Altough I never did a Catholic confession (even though I was born Catholic), it seems priests were therapists in the modern sense.
Stavrogin's character is the amoral man used by Dostoevsky.
Modern psychologists will tell you that emotional pleasure does not necessarily have to come from doing good. Getting a response - any response - is a satisfaction to the human psyche. While some therapists think this is "normal", they require you take responsibility for the act - and the consequences. Stavrogin does not.
In this sense Stavrogin - in modern non-religious terms - could be called a sociopath or psychopath.
Dostoevsky was indeed very astute and ahead of his time.