Somebody else here mentioned he was autistic...and some people with autism are affected by sounds.
I honestly think it was just SOP when he went to the range with his mom...so he stuck them in without even thinking about it.
I've been working with Autistic children for more than 16 years now. I have never come across a case, either personally, or through any reports by others, of a person with Autism engaging in violence of this nature. The media have done a huge disservice to people with Autism by their early reporting regarding this killer having this disorder.