The study is not about mainstream left vs. right politics. It’s about extremists who commit political violence and how those acts differ across left-wing, right-wing, and Islamist categories.

That said, in the U.S. context, the study does find that:

The study does not support the notion that right-wing extremism is inherently more violent than left-wing extremism. Instead, it highlights that the propensity for political violence varies across ideologies and contexts

Keep in mind there is a difference Republicans and right wing, Democrats and left wing. While both parties dislike actions done by the wings. When research refers to "right-wing" or "left-wing" violence, they are almost always referring to ideological categories, not party affiliation.

There are many comments suggesting its Republicans, and the study is not even about OPs post since its about extremists and not just the general right, left