Antifa, the left wing group, uses violence often in its methods.
https://nypost.com/2019/07/17/liberals-cheer-as-antifa-violence-escalates/What about the Sanders supporter who tried to kill Republican congressmen playing baseball. Five people were shot.
https://www.history.com/this-day-in-history/james-hodgkinson-shooting-republicans-baseball-gameThese were only two of many left-wing groups and terrorists dating back decades including the Black Liberation Army, The Weatherman and The Symbionese Liberation Army, just three that I remember. Many of these groups even used bombs in their violence and assassinated cops regularly.
Black Liberation Army
A splinter group made up of the more radical members of the Black Panther Party, the Black Liberation Army (BLA) sought to overthrow the US government in the name of racial separatism and Marxist ideals. The Fraternal Order of Police blames the BLA for the murders of 13 police officers. According to a Justice Department report on BLA activity, the group was suspected of involvement in over 60 incidents of violence between 1970 and 1980.
Symbionese Liberation Army
The Symbionese Liberation Army (SLA) was an American self-styled, far left "urban guerrilla warfare group" that considered itself a revolutionary vanguard army. The group committed bank robberies, two murders, and other acts of violence between 1973 and 1975. Among their most notorious acts was the kidnapping of the newspaper heiress Patty Hearst.
Weathermen
The Weather Underground Organization was a far left organization active from 1969 to 1975. It originated in 1969 as a faction of Students for a Democratic Society (SDS)[23] composed for the most part of the national office leadership of SDS and their supporters. The group collapsed shortly after the U.S. withdrawal from Vietnam in 1975.
The point is, America has a rich history of single nut jobs of all persuasions as well as violent groups from one end of the political spectrum to the other. These groups have long pre-dated Trump. Wacko Americans of all stripes don;t need much to get them going.