Corporations aren’t causing a mass extinction just for shits and giggles, they’re doing it because billions of individuals buy their products and services. If the billions of individuals stopped buying it, the corporations would stop making/offering it. The rich cause more harm in the short term, but even poor people having more kids despite the biosphere not being able to sustainably support even a fraction of the current population, are more omnicidal in the longer term.
Billions of individuals choose to eat animal products giving money to factory farmers and industrial fishing companies - 2 industries that cause more pain and suffering than all other atrocities ever committed in all of history, combined (1-3 trillion fish are tortured to death every year by fishing companies, and at least hundreds of billions of animals are enslaved in torturous conditions in factory-farms every year). I live in a 3rd world country, and went vegan almost 20 years ago. For the people causing most harm: those in rich countries, it’s easier to be vegan.
For those who can’t grow their own plant-food, there’s still this: https://ourworldindata.org/grapher/land-use-kcal-poore
It’s both: “No single raindrop believes it is to blame for the flood.”