My suggestion 

'A bola da vez'

(similar in English could to be ' the cue ball').

Reference to the snooker game where the balls have a defined sequence to be pocketed, and after pocketing the cue ball does not return to the game.

In denouncing the killer he became the cue ball or [a bola da vez].

João está com receio de ser a bola da vez (the next) em ser demitido.

John is afraid to be the cue ball/ a bola da vez/ to be fired.