Yes...inmates killing while on the inside are included.
As for the Wapo article, since they seem to intimate that the Pittsburgh Synagogue murders were right-wing, despite the accused gunman calling out Trump for his ties to Israel, I doubt the accuracy of this study.
My particular favorite:
North Judson, Indiana, March 3, 2017. Aryan Circle member Edward Blackburn allegedly shot and killed another man who was reportedly dating his ex-girlfriend.
Concur.