The "I am that guy" trope

The trope namer is, of course, season 3 episode 6 of The Expanse. Amos persuades Dr. Prax to not shoot the villain, saying "you're not that guy", but as soon as Prax is out of sight, Amos says "I am that guy" and performs the deed himself.

I just noticed that the movie >!Supergirl!< ends the same way.