That claim is founded on some various things he used to say back when he was around. Here's a quick collection of screenshots from a cursory search on the matter.
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Notably to the best of my knowledge he had never explicitly stated having feelings of necrophilia or pedophilia, but when you have statements such as "I don't see the issue with beating off to dead children" and "I have pictures of dead children on my computer" it doesn't take much of a leap to put the two together and come to the reasonable assumption that he very well might have beaten it to said pictures.
Whether he actually did or not has never been confirmed, so whatever conclusion one may wish to draw from this mostly just depends on the preconceptions one might have already. Important to keep in the back of your mind as well, is that this was a long time ago now might not reflect his person currently, or he might have been purposefully provocative back then, who knows. Either way it does not look good on him these days.
Interestingly besides this he was known in the community as a mathematically gifted person, which is usually the second thing people bring up about him. You may see some parallels there between him and notable pedophile Hinds who was also a mathematically inclined person. Much to consider.
