From Fox News:

The report by state police also mentioned pedophilia, as investigators said they found on Lanza's computer a file they described as "advocating pedophiles' rights and the liberation of children." Investigators also said they found a screenplay describing a relationship between a 10-year-old boy and a 30-year-old man.

Here's a good infographic thing with a lot of detail, most the right side is relevant. Here's a more fleshed out theory with links. Summary - There's a report Lanza attended St. Rose church (which also has a school he briefly attended around 5th grade) less than a mile from the elementary school. A pedophile priest was assigned to that church when Lanza was around 6-9 years old, right before he was caught in sex sting at his next assignment. One of his 5th grade classmates said the following:

Lynch says he remembers watching a sex-education movie, and Lanza saying he was about to throw up and needed to leave the room.

It looks like the dots are starting to connect. I wish the media was put the spotlight on this Castaldo creep.