What other risks are there that are directly intentionally human driven that could cause such an extreme amount of destruction, death and societal collapse over the span of a few hours?
Not sure why you added the "within a few hours" qualifier or "intentionally human driven". Have you already thought of something that takes longer, is done by humans accidentally, or is natural? Is it climate change? Does that already answer your question?
There is nothing else so swiftly and completely destructive that is entirely driven by humans and their choice as to whether they should destroy or not.
That's the thing about nuclear war, unlike all those other things, it really comes down to a few humans deciding not to do it. That's what makes it more terrifying than those other ones.
But asteroid or comet impacts. Comets in particular can come so fast and undetectably that we won't have time to react. We could prevent a comet impact if a few humans decided to build such a defense system, and we might be wiped out if those few humans decide not to do it. Sounds equivalent to nuclear war.