Well, we don't know the motivation. That said, operating on the spouse-usually-did-it level of police hunch, it appears he targeted Republicans on purpose, so you are probably correct this is an unhinged leftist.
And if the motivation turns out as it initially appears, yes, the MSM and Democrats spouting hate and lies are partially to blame. As the fringe right tends to attract unorganized angry white men, the fringe left tends to attract organized violent social justice warriors.
And, despite all the hype about angry white men, they tend to be pretty law abiding (although loner exceptions abound), while the violent left is more widespread and organized.
Remember, actual political assassinations tend to be from leftists in the USA: Lee Harvey Oswald was a card-carrying communist mad at what-would-be-now a moderate (probably Republican) John Kennedy. The fat loser who shot Reagan -- another violent liberal loser. Heck, even Squeaky Frome (of Mason Family fame) who shot at Ford, while mainly just a nut, espoused hippy nonsense.
And the absurd violence against Jewish people and anyone conservative (and especially conservative Jewish people) on college campuses is an organized leftism phenom.
So, yes, odds are, he'll turn out to be a Democrat.
And, yes, the fact that Time/Warner/CNN sponsor a play about the assassination of Trump, the fake "hands up, don't shoot" lie, and the other evil and violent nonsense played up by the left has created a really piss-poor environment, ripe to encourage violent acts like this.
They've been very irresponsible (as has Trump, in other ways) in creating a culture that very well could fall apart.