when they aren't born ill then what factors contribute in making them a rapist?

First a 5 year old and now a 13 year old girl raped. Why do people rape kids? I mean a 5 year old and even a 13 year old are not sexually attractive. Moreover, it seems like a universal thing. 
From Asia to Europe, from Africa and Americas, it happens everywhere. So the fault isn't exactly the culture of any particular place, as far as I can see. One thing I am sure of is that the people who do such things are surely psychologically ill. Some of them are born ill most probably but isn't there some way to rectify this? Still a more important question, when they aren't born ill then what factors contribute in making them a rapist? And what are the action that could be taken to stop such factors from appearing in the society?