"If God does not exist, everything is permitted."
I see this as a progression of Evil. Sort of like we think of gate way drugs. Start with tobacco, go on to booze then pot, and on to hard drugs to keep getting you higher. Comes to a point that if one gets at the top of the ladder they die.
Sex is the same thing really, sex is an inborn addiction that all living things have. So some people keep needing more and more stimulus to get off and experiment looking for the ultimate experience. When one gets bored with same old same old heterosexual sex the temptation to boost it up a level, a hooker, another male, then when they hit rock bottom to animals and children.
This is a sex addiction same addiction as drugs. Got to get some satisfaction. Drug addiction can be cured but not sex addition.
Is this a mental disease --- I think so, these people are phycopaths that care about no one but their own selves and their own gratification of their addiction.
Their world revolves about their addiction, on the outside they are normal to others but when the addiction kicks in, then we get the serials killers and the monsters that rape and kill children.
When people complain about the death penalty, ask them if they would be willing to allow at any amount of money to take these people into their own homes be they family or strangers ?????
Nothing can cure them, they are a danger to others and will always be.