A note: this data doesn't say what you want it to. It gives the percentage of white-supremacist killings out of all right wing extremist killings.
Slide 4 of this: https://polisci.barnard.edu/sites/default/files/inline-files/Emily%20Kramer%20-%20Right-Wing%20Extremism%20and%20Mass%20Shootings%20in%20the%20United%20States.pdf
gives a percentage over years of right wing vs. left wing vs. Islamic extremist mass shootings.
