I haven't seen this mentioned yet, but you're required to register a party affiliation before you can vote in the primaries. As a liberal, I've considered registering Republican in situations where the Democratic nominee has pretty much been decided and I want to give a boost to less polarizing Republican candidates. Pennsylvania is one such state that uses the closed party system (you need to be Republican to vote in Republican primaries). I've since moved to a state that allows "Independent party" members to choose the primary they wish to vote in.

Not saying this explains the shooter's party affiliation, we still need to wait on all the details. Just something to keep in mind so we don't jump to conclusions.