of all people... (surprise)
Expresses surprise or disapproval that someone of a certain status or character would do something unexpected.
This pattern points out that someone, given who they are or their position, should not have done something—or did something surprising. It's like saying 'Of all people...' or 'For someone who is supposedly...'. The speaker expects the person to act according to their status, and the pattern highlights the contrast between that expectation and reality.
You use it when you want to express shock, criticism, or disappointment. The noun before it is often a profession, title, or role that carries responsibility, like 'teacher', 'doctor', or 'adult'. It is a somewhat formal and literary pattern, often found in written opinions or news, but can be used in speech for strong effect.