the system allowed for them to accumulate enough wealth and power

The system they designed.

Everything functions as designed in a capitalistic laissez faire system, that is until a critical point when it won’t.

Per their design.

"It's a cancer, so we should find a way to deal with the genetic cause of the cancer in our patient rather than cutting it out?"

Thank fucking Christ you're not an oncologist, because your own metaphor shows why you have to get rid of the billionaires immediately. First, cut it out if you can. Second, if it starts coming back, treat it with poison and hope the body doesn't die. Third, find a way to treat the genetic defect or environmental factor which caused it in the first place. You have to do all of those, but the first one is objectively more pressing.

If I kill one billionaire, and another shows up for me to kill again, and I can, physically, do that, I will kill them all until I fall dead of exhaustion or there are no more billionaires.