I would be very, very careful about asserting who drew "first blood" the left or the right. Apart from there being not much out in the public domain yet (it is still very early in whatever investigation will be done), there is also the issue of what the timeline is. When are people starting to keep 'score' on which side the perpetrators of political violence were/are on? If the scorecard is started as of today, and the shooter was left/left leaning and did so for political reasons, then scoring 'first blood' for the left could be accurate.
The unfortunate reality is that whilst political violence has not been a common occurrence in the US, it has been happening periodically for decades, by both the Left and Right. The Weathermen (far Left Marxist militant group from the late 60's and well into the 70's) carried out several bombings in that time. Starting around the same time was a loose Far Right organization known as Posse Comitatus (yes, like the Reconstruction Era act) which was essentially the forerunner to many of the sovereign citizen and/or "Christian nationalist" groups active in the US since then. The Montana Freemen group active in the 1990's, which included members getting convicted and sentenced to prison for engaging in terrorism, are one such descendant group. The single deadliest domestic terrorism incident in the US remains the Oklahoma City bombing of the Alfred P. Murrah Federal Building by a pair of Far Right anti-gov't extremists in 1995.
One of the major concerns about Far Right violence at present, is the volume of threats and the degree that it has become 'normalized' and apparently acceptable.
Side note: A friend of mine has a reporter friend who was on or near the stage at the time of the shooting and from pictures it looks like Trump might have been hit a 2nd time, in the upper right chest, with the round being stopped by body armour. I cannot confirm the veracity of the pictures, but it seems quite possible.
At this point, I am concerned that certain elements will now be even more 'amped up' and being seeking to 'even the score' possibly by engaging in or expanding political violence.