Reading about past pandemics, like the Spanish flu, it looks like, the cytokine storm was likely the issue there as well, from an 2018 article below.
They great thing if leronlimab is approved, this could be used on future pandemics as well.

https://www.bbc.com/future/article/20181029-w...-so-deadly

Besides replicating very quickly, the 1918 strain seems to trigger a particularly intense response from the immune system, including a ‘cytokine storm’