The data is clear that the lion share of mass shootings have been perpetrated by people who have right wing political view. But even then, that really isn't a meaningful statistic. Political view point is most relevant if there is the potential for the shootings to be part of a political movement. That has happened with the Brevik mass shooting spawning a number of similar attacks, including Dillon Roof shooting at the black church in SC and the Texas Walmart shooting. These shooters were carrying out mass shootings as part of a white supremacist movement to try to spark a race war. On the left, there really hasn't been any real organized political violence sine the days of the Weather Underground. A few recent shooting have been carried out by trans or queer shooters. But other than the Charlie Kirk assassination, there has been only some vague online chatter about some sort of militant trans/queer movement. But no real connection between the shootings and that sort of online stuff.
What we are really seeing with mass shootings in the US is nothing more than a continuation and amplification of what used to be called "going postal". Someone with various degrees of mental health issues goes over the edge and lashes out with a mass shooting that ends in either suicide or suicide by cop. The recent Michigan and NC shooters were both veterans who had histories of suffering from PSTD. The Michigan shooter was reported to believe that the LDS followers were the antichrist. The motivation for the shooting is almost irrelevant as it is often just a scapegoat for whatever psychosis is driving them over the edge.
That then leads to the inescapable issue of why does this happen in the US all the time, but almost never happens in any other country (save an except for countries that have gang warfare, sectarian violence, etc.). This year, the EU had three mass shootings. Three. In the US, we are up to about 325+ with 300+ dead and over 1,300 wounded.
What is are the big differences between the US and the EU and most other countries? First and foremost, guns are freely available to anyone and everyone in the US. And with the exception of fully automatic machine guns (but bump stocks can get you similar firing), you can get any kind of firearm with large capacity clips. Anyone in the US can easily equip themselves to be a killing machine.
And in the US, mental health care is largely a privilege. In countries with universal health insurance and strong employee benefits, mental health care is far easier to access and benefit programs protect people who cannot work due to mental health issues. In the US, most people do not have access to mental health care and many will even go as far as avoiding seeking help for fear that it may impact their ability to work or factor into custody fights, etc.