While there's some element of truth to what you've said- the DNC is using the "End of Democracy" meme as one of their primary themes in their campaign messaging- I don't think that had anything to do with this shooting.
Statistically speaking the vast majority of domestic terrorism comes from far-right extremists. We don't know the motivations behind the shooter at this point in time, but the consensus seems to be so far that he was conservative.
I think it's much more likely he was radicalized online and either
a) wanted to kill Trump to further polarize the political tension in an accelerationist sense (sort of like how far-right organizations were shooting both protestors & police during Euromaidan. Just to increase the outrage, the violence, etc).
Basically an attempt to weaken the regime
Or
b) he became a white supremacist and those tend to hate Trump. Go on stormfront forums and see what they say about Trump. They think he's a jewlover, fake messiah, etc. They hate Jews, Trump loves jews. Therefore, Trump is a potential target.
Or
c) was just nutty plain and simple. Ie killing Trump would unite him with some cartoon character from Nickelodean in the afterlife.
All of those are vastly more likely than someone being radicalized by DNC messaging, I believe.
It's sort of legally gray but generally speaking in the US downloading is a civil offense but not a criminal one. You can get sued by the copyright holder for example but you won't end up in jail over it.
People usually never get sued for it because it's not worth it for Comcast to pay for lawyers to try and extract any money out of regular people. Not only will they almost certainly be unable to even recoup the lawyer fees, they risk getting a lot of bad PR for no gain.
What's usually considered an arrestable offense is uploading aka distribution. Once you start hosting seedboxes then you enter the area where you're liable to go to prison.