I read this fascinating account of Lopez Rega's life (in Spanish) from cover to cover in a couple of days and found it riveting.
He was known as El Brujo and quite how he got under Peron's skin in the 50s is anybody's guess. There was definitely something odd going on there, as well with Isabel Peron and the body of Evita.
He seemed to cast some kind of spell on Peron and ended up becoming incredibly powerful, setting up the Triple A and losing control of the violence (which he encouraged) completely.
here's a very good documentary on Lopez Rega which is well worth watching.