would you not call that "being under attack", which you brought up as a justification for homicide?

No, with "being under attack" in this case I meant immediate, impending physical harm with probable lethal consequences. Self-defense in that sense should strive to be somewhat proportional to the severity of the attack.

But you raise an interesting point:

We absolutely should do something about the suffering arising from the greed and cruelty of the super-rich. The difficulty with removing individuals is that the institutions propping them up will continue to exist. While their ownership (and the mechanisms of inheritance / transfer of that wealth) as well as the attendant authority is accepted as legitimate, the problem will continue to exist.

The theoretical approaches to changing this system – whether from within or without – don't strictly require violence, but the people who believe in that legitimacy will follow orders to defend it against people that would render those orders void. If they do so violently, it may be necessary to defend ourselves.

And this is where you have a point I didn't originally consider: if we perceive the orders (and thus the ones giving them) as the ulterior enemy, self-defense could extend beyond the immediate threat of people misguidedly following them.

This could also be applied to, say, healthcare execs that make decisions with significant impact on people in need of lifesaving care, or military industrial cronies.

Whether responding with violence is a good idea or at all effective is a different question, but I can see an argument that targeting key figures behind life-threatening orders would at least be a legitimate form of self-defense.

But just to be clear, my first comment was meant as a joke.

That apparently went over my head, but it lead to an interesting line of thought I didn't consider before, so I'll consider that a win.