As more cases of COVID-19 are studied and treated world-wide, it had become apparent that lethal and most severe cases of pneumonia are due to an out-of-control inflammatory response to the SARS-CoV2 virus. I explored the putative causes of this specific feature through a detailed genomic comparison with the closest SARS-CoV-2 relatives isolated from bats, as well as previous coronavirus strains responsible for the previous epidemics (SARS-CoV, and MERS-CoV). The high variability region of the nsp3 protein was confirmed to exhibit the most variations between closest strains. It was then studied in the context of physiological and molecular data available in the literature. A number of convergent findings point out de-mono-ADP-ribosylation (de-MARylation) of STAT1 by the SARS-CoV-2 nsp3 as a likely cause of the cytokine storm observed in the most severe cases of COVID-19. This may suggest new therapeutic approaches and an assay to predict the virulence of naturally circulating SARS-like animal coronaviruses.