I loved this movie, the theme, the acting, the script, all is excelent. Is a really sad movie, all the characters are victims in one or other way, the girls that theo raped, theo, nettie...
But the question is, can a rapist be "cured"? Seems that experts think that is no possible, but still is on debate.
Cured from what? From a lack of ability to control his urges? Personality might be a considered constant by psychology, but behaviour changes dependent on many variables, both internal and external.
Just rapist behaviour can emerge in the course of a lifetime it can be controlled, ofc. If you deny that it can be controlled, then this denies humanity their inner creative power, their free will, their responsibility for their own actions. It's a strongly philosophical question and while I don't think you'll find the answers you seek on here, I'm sure this has been heavily pondered by both men of law and of philosophy.