I have some good news and some bad news.
Bad for America, good for the rest of the world.
If you are limiting the crash to just the S&P 500, please recall that this is basically an index of American greenback-denominated stock. Money is money, and it pursues the highest return. If there is a stock market that is offering higher returns, then the money will flow out of the S&P and into this other market. There are many other stock markets besides the American stock market.
Consider what happens if Covid mutates into a more deadly form, hits America the hardest, and America takes just as disastrous and divisive a stand as it took against the first Covid assault. American corporate firms are heavily hit. Top executives drop like flies. There is a corporate leadership vacuum, and American S&P corporations fold like a deck of cards. Imagine Bezos, Musk, Zuckerberg, and other chief executives all dying within months of each other.
Money would flee the S&P, and seek out safer havens. If, say, Europe or Asia were to keep the pandemic under control, and their corporations withstood the disease, the money would all flow to European stock markets.
Now add climate change and climate disasters to the mix. America is already experiencing supply chain disruptions, imagine if all transportation shut down. A gasoline shortage, trucks off the road, a few major rail lines destroyed in key places by landslides and such, the bankruptcy of a few major airlines, and an American populist runs again for President for the 2024 elections and looks like he might win, and control of Congress and Senate becoming stalemated in a tie in 2022. Nothing gets passed, spending bills are left hanging, and the American government is left stagnated and completely impotent.
Now add in an assassination of the American President, a successful riot against the American House, killing a huge swath of legislators. This time, they come with guns and ammo, and it is between the supporters of the assassinated President and the side that assassinated the president. Consider if Pence HAD been assassinated.
Now, add in the successful assassination of the Pope, and it is speculatively falsely traced back to an American Antifa terrorist organization because the American Supreme Court took an extreme right path on abortion.
A perfect storm for the corporations listed in the S&P.
The sad part is, absolutely everything in this scenario is plausible, because it has happened, albeit less severely and in individual cases.
TL:DR
It will not be an isolated, single event, but a juxtaposition of many, many converging scenarios.
Oh, and the American Civil War never DID end, it just paused in a truce.