Clockwork Knight
Arcane
Somehow, miraculously you survive, at this point the average waster would be thankful to be alive, the courier instead unleashes a rampage across the wasteland against all odds. That to me, is unbelievable.
It's almost as if the PC isn't an average wastelander.
(Besides, the lone wanderer does the exact same thing, and s/he doesn't even have revenge as a motivation since Dad left of his own accord and the Overseer is the one to blame for the murder spree in the vault. You don't have the option to kill Dad out of abandonment butthurt, so basically an unhappy PC will just seek him out to give him the middle finger)