Justice is an eye for an eye
You're just being stupid David, as i can prove:
"Say someone has a coughing fit and runs someone over, or knocks a plant-pot off a windowsill and it hits someone in the street below."
They can get sued now and lose their house and job for those circumstances, accident or not.
Doesn't require an eye for an eye unless the person did those things on purpose.
"What if two people have a minor argument which ends up with them each poking fingers in each other's chests, and one drops dead - does everything depend on making a decision about who started the argument?"
Wouldn't break any laws so how could they sue or go to court?
Assault only works if there's evidence so a poke would have to bruise or cut someone, in which case it's obvious who came off worse.
Killing someone by poking would not equate to murder in any circumstance
"Do you think you could get even half a dozen people to agree what a fair blame threshold is?"
It's called law and more than a dozen people have agreed on that for thousands of years
"What if the guy who then falls into the machine was doing something the manager had told him not to do?"
Then justice was served and that's also a deterrent for other workers not to do it
Use your brain David