This has been pretty thoroughly debunked by now. The kid was a conservative through and through. Why he'd shoot the presumptive nominee of his own party is a mystery though, and we might not ever find out.
It's pretty common for people to vote in the other party's primary (which is what registers you in most states) when they've got an incumbent from their own.
If they view their own primary as unimportant, they can register with the other party and vote for the least-objectionable (or, more nefariously, the least-electable) candidates.
I registered Republican this year because the Dems in my primary were already locked in. So I'm technically a registered Republican even though I've never voted Republican in the general election.