A manager with a sense of humor is different from a manager who jokes about ending your employment. Imagine getting pulled over and a cop jokes about arresting you- are you laughing or do you feel unnerved? You were being sarcastic, but unnerving employees by making jokes rooted in your power over others does have the potential to negatively impact company culture and thus the company itself. The power dynamic matters.

Making jokes about firing subordinates shows a bad sense of humor -- it's not funny, it's making light of a power imbalance at the expense of the subordinate.

It's like when a cop makes jokes about shooting you after pulling you over for expired tabs (happened when I was a teen).

It is more or less universally recognized that jokes are only funny if they are punching upwards. Someone in a position of power making jokes about using that power against someone with less power is more akin to bullying.