Livelsberger's uncle said "He loved Trump, and he was always a very, very patriotic soldier, a patriotic American." He was also very dissatisfied with the government but I don't see strong indications that he was criticizing from the left.
Crooks donated once to ActBlue. He was also a registered Republican and the FBI deputy director described his social media activity as anti-semitic and anti-immigration. Again, case seems open.
Routh is certainly reasonable to classify as a leftist.
> well, I'm not sure "left" is the right word, but more generically not-the-Right Americans with guns who I would categorise as an "extremist".
Despite who and what they targetted.
For a mere protest-based example of oppositon: an ex of mine is an actual literal communist, so way to the left of the Dems in a lot of regards, and yet was still so anti-Dem as to promote Trump the first time around.
The "anti" part seems more relevant than the "pro", when the question is violence.
But perhaps that's just me.