If my memory is being good today, Lanza took up DDR as a great fixation of his recreational time in part because he had a rare issue with his head that reduced his sensitivity to pain, causing his body to be able to endure extreme levels of exertion without any physical fatigue, and so he’d be seen at local DDR stations in arcades or movie theaters dancing for hours non-stop almost every day, which allowed him to get really good at it. Here's a video of him playing it in a theater.
Additionally, I recall reading that he had a DDR machine at home that he'd play at least once every morning right after waking up. He's said to have had a lot of peculiar hobbies. I'm pretty sure that, prior to the shooting, he also studied Esperanto and practiced the erhu (a Chinese string instrument, kind of like a violin,
here's an example) in his free time.