Monty Burns discovered that Waylon Sr.'s death was not for an accident, but a murder. Alfred Spencer killed him, he sabotage the system. For years he felt responsible for his demise, but Smithers told him the truth about the incident. An awful pain Burns feel in that moment when he remember that death, along with hate towards Alfred.
C. Montgomery Burns and Waylon Smithers Sr. (c) Matt Groening Alfred Spencer (c)
Just wait for it, Alfred. I'm not done with you yet.