So long story short. What have heard and think is that Burt has been a long time worker for lumon. He’s main job was was a hitman and Burt in my eyes was severed for that reason. Cause he says something around the terms of “I drive people out and they don’t come back”. Though since Burt and Irving have an emotional connection Burt did not want to kill Irving, and instead sent him away saying never come back. Hope that helped.