Possibly because of the way the question was asked. It could be interpreted in the same sort of manner.

How do you know for certain that the shooter was a Rep? Are Reps only allowed to register as Reps and Dems as Dems? Can a Rep vote for a Dem and vice versa? What if you were registered as a Rep and want to now register as a Dem? What requirements are there to make the change, or is that not possible?

What about in general outside of politics? Can people sign up for whatever they want to for the most part, (beyond passing a test or having the qualifications if so required?) If they sign up for something, can that only mean they are deeply involved in it, that they must like it, that they must want it to thrive?

From another point of view, the question should read, What evidence do you have that liberal rhetoric is why the gunman shot at Trump?

Plus, we also know the kid was heavily bullied in high school.

I think that when it comes to being republican or democrat the most clear definition of this is which one you are officially registered with. After that information is known I'm going to need some evidence to the contrary to entertain the idea that he's not really republican. 

There are two competing theories for why this dude shot at Trump. Either he's a political radical pushed forward by liberal rhetoric, or he's a school shooter type who chose a political target instead of shooting a school. I'd argue the facts as they stand point towards the latter. 

Registered republican

donated 15 bucks once to progressives

had photos of several political figures on his phone (I'd argue targets)

To me these facts all point heavily towards a school shooter at which point there's no reason to think liberal rhetoric had anything to do with it. For the other theory to work the shooter would have to have registered republican just for fun and although the pictures of Trump on his phone were for targeting purposes the pictures of several other politicians is just because he likes their faces. No manifesto, no online presence, no history of political activism. There's just nothing to point towards this being liberal rhetoric other than the dude shot Trump which is kind of circular logic, like there's no other reason someone could possibly shoot at Trump. 

How much did he donate to Reps? You're telling me a Rep donated to Dems but gave nothing to Reps at all?

How many presidential assassin types decide to shoot up schools instead?

Even if by some chance he was a school shooter and decided to go after Trump instead, which is really really reaching, the question is why choose a politician as a target, who's not just any politician but a past President, presently running in a campaign for President? How does the gunman's problems, whatever they may be, cause him to likely be a school shooter, whether those problems are directly school related or not, and yet lead him to try and assassinate Trump?

Personally my best guess at the moment, which is just a guess without more evidence, is that based on the fact that it looks like the shooter had potential Rep and/or Dem targets in mind, that the problem is rhetoric period. You could argue about which side is worse, but it would mostly be a waste because both sides will stand firm against the other.

I personally just want to see legit news reporting and both candidates pointing out each others strengths and weaknesses as well as future plans, and leave out all the BS nonsense before it goes to far, because it was inches from too far already.

The donation means nothing. It happened several years ago, and he was 17. Young men frequently change politics during that age. 
I was more conservative when I was 17 for example.

The evidence points to him doing this to get attention. 

He was looking up schedules for Joe Biden and Donald Trump, Donald Trump happened to be the closest.

If the donation means nothing, then why should the registration mean anything either? How frequent is frequently exactly? If young men change their minds so often, how do we know he hadn't changed his position since? Plus, again, it doesn't matter if you sign up for something, there's nothing that guarantee's you're in favor of that or want it to succeed and thrive.

The evidence as to why is pretty wide open. That's why others have mentioned theories about why. Your take is one possibility however.

Another would be that rhetoric from politicians, media, and even the public, from one or both sides, led him to believe certain people needed to be targeted. If he was looking at Trump and Biden, the gunman would likely assume it would be extremely difficult to make a getaway, so if you're only going to get the chance at one target, you'd attempt to take out the biggest perceived threat first.

I think that when it comes to being republican or democrat the most clear definition of this is which one you are officially registered with. After that information is known I'm going to need some evidence to the contrary to entertain the idea that he's not really republican. 

There are two competing theories for why this dude shot at Trump. Either he's a political radical pushed forward by liberal rhetoric, or he's a school shooter type who chose a political target instead of shooting a school. I'd argue the facts as they stand point towards the latter. 

Registered republican

donated 15 bucks once to progressives

had photos of several political figures on his phone (I'd argue targets)

To me these facts all point heavily towards a school shooter at which point there's no reason to think liberal rhetoric had anything to do with it. For the other theory to work the shooter would have to have registered republican just for fun and although the pictures of Trump on his phone were for targeting purposes the pictures of several other politicians is just because he likes their faces. No manifesto, no online presence, no history of political activism. There's just nothing to point towards this being liberal rhetoric other than the dude shot Trump which is kind of circular logic, like there's no other reason someone could possibly shoot at Trump. 

None of these things are "proof" is what I meant. 

The registration isn't proof of him being Republican, and the donation isn't proof of him being Democratic. I would argue that the registration is more meaningful as evidence because it is more recent. 

When I say frequently, I don't mean an individual changes their mind multiple times. I mean young people often change politics when they become adults. Not that they swap between the two on a weekly basis. 

There's probably no way to know the absolute truth, short of finding a manifesto or something. But certain theories are going to be more likely to be true or less likely to be true based on what we know of the real world and what we know of the shooter. 

There's basically one 3 year old piece of evidence that suggests he's a liberal, and several more recent pieces that suggest he's a conservative, and even more recent information that heavily points to him not doing these things out of political motivation.