So basically you read a list like the OP posted (with zero sources and no further research on your own to confirm any of it) and other than the ONE attack that can be linked to a partisan attack, and your conclusion is you have no reason to believe that the gunman are NOT leftwing nutjobs. Got it.
I guess by your logic I can just assume because the Parkland High shooter had ties to right wing white seperatists and Democrat Gabrielle Giffords' shooter was big into the Tea Party that all mass shooters are racist, right-wing nutjobs.
But something tells me you would take issue with how I came to said conclusion.