One of the main hypotheses for deaths in Covid-19 is that the virus elicits a very strong immune reaction from the body. The strong immune reaction ("cytokine storm") is meant to defeat the virus, but unfortunately also damages the body, causing multiorgan failure leading to death. As you can see from the extracts below, cytokine storms are also hypothesized to be a cause of death in other viral infections such as SARS, MERS and influenza.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30183-5/fulltext
Early studies have shown that increased amounts of proinflammatory cytokines in serum (eg, IL1B, IL6, IL12, IFNγ, IP10, and MCP1) were associated with pulmonary inflammation and extensive lung damage in SARS patients. MERS-CoV infection was also reported to induce increased concentrations of proinflammatory cytokines (IFNγ, TNFα, IL15, and IL17). We noted that patients infected with 2019-nCoV also had high amounts of IL1B, IFNγ, IP10, and MCP1, probably leading to activated T-helper-1 (Th1) cell responses. Moreover, patients requiring ICU admission had higher concentrations of GCSF, IP10, MCP1, MIP1A, and TNFα than did those not requiring ICU admission, suggesting that the cytokine storm was associated with disease severity. However, 2019-nCoV infection also initiated increased secretion of T-helper-2 (Th2) cytokines (eg, IL4 and IL10) that suppress inflammation, which differs from SARS-CoV infection. Further studies are necessary to characterise the Th1 and Th2 responses in 2019-nCoV infection and to elucidate the pathogenesis. Autopsy or biopsy studies would be the key to understand the disease.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4711683/
Severe influenza remains unusual in its virulence for humans. Complications or ultimately death arising from these infections are often associated with hyperinduction of proinflammatory cytokine production, which is also known as ‘cytokine storm’. For this disease, it has been proposed that immunomodulatory therapy may improve the outcome, with or without the combination of antiviral agents.