It is easier to convert such criminals to Islam than to Christianity. Radical versions of Islam pander to human passions and violent people can make fierce warriors.
Christian conversion involves contrition, humility and love of enemies. It can be much harder for hardened souls.
Islam is made to order for people with sociopathic tendencies, just like Nazism. (Mad Mo was probably a sociopath himself). Criminal activity is no longer deviant but can be justified as a form of jihad.