Many paedophiles were themselves abused as children. For many I think it is nurture, or the lack of it, rather than nature. It is noticeable when you read court cases involving very young paedophiles, 16 - 25, how often the perpetrator is described as having difficulties developing adult relationships because of problems in their own childhood. However I am sure for some, probably the most persistent, it is in their nature and they are born with those inclinations.
I think that the availability of pornography of the worst kind on the internet has hugely exacerbated the problem. Men who start with 'ordinary' pornography and, perhaps, for many, then get drawn into looking at more extreme varieties; paedophilia, extreme violence to women etc. may move on to physical abuse when, without the ready access to extreme pornography, this will not have happened.
I do not think that looking at the images on a screen necessarily means that they might be tempted to physically abuse a child themselves (although children have been abused to make the images), anymore than most people would actually want to act out their sexual fantasies, but looking at images does increase the likelihood of offending and those doing it should be prosecuted.