I can definitely agree that rape is about payback and revenge. There exists a type of rapist who has been raised by an abusive single mother, unable to form healthy relationships with the opposite sex, and who uses rape to avenge himself of his mother and womanhood in general.
I think we should look at the time a person seeks treatment for pedophilia. If someone comes to a psychiatrist's office or checks himself into a mental hospital, and states that he has violent sexual urges towards children and fears he may not be able to control himself much longer -- well, this is the type of person who needs and deserves our help, and can actually benefit from it.
On the other hand, if a pedophile is caught after years of abusing who knows how many kids (therefore causing them irreparable trauma), and he then throws up his hands and says: "I'm mentally ill! I need help!", well...
I think the reason pedophiles are hard to cure simply because what they do gives them pleasure, and because it is still, despite everything, easy to get away with doing what they do. Children don't vote, they don't pass laws and they don't pay taxes. Pedophiles do.
This is a very hard question. The fact is that the recividism rate for pedophiles, especially those focused on children of their own sex, is staggeringly high. I think at the moment it is more important to contain this phenomenon than to debate about what causes it. To answer your question, people under 18 can be monsters too, and letting them loose in society will only cause harm -- as terrible as that sounds.