At what point can a group of citizens sue their government/ leaders for endangering their lives?

[The Velveteen Ocelot]

5. They can't. There are legal doctrines of sovereign or discretionary immunity

that protect government officials against liability for harm arising from policy decisions made in their official capacity.

The FTCA [Federal Tort Claims Act] (and most state tort claims acts) preserve immunity from tort liability for the discretionary acts of government employees. This discretionary function exception is perhaps the most notable and complex exception to FTCA liability.

A discretionary function is an act involving an exercise of personal judgment. The basis for the discretionary function exception to the FTCA is the legislative branch's desire to prevent judicial second-guessing through tort actions of legislative and administrative decisions grounded in social, economic, and political policy. The federal government retains immunity from tort liability for itself and its employees for the performance or nonperformance of discretionary functions. This immunity is granted when the act in question requires the exercise of judgment in carrying out official duties. Discretionary immunity applies unless a plaintiff can show that a reasonable person in the official's position would have known that the action was illegal or beyond the scope of that official's legal authority. Harlow v. Fitzgerald, 457 U.S. 800 (1982).

https://biotech.law.lsu.edu/map/DiscretionaryActs.html