if police are about to shoot, and have good cause to, then it would seem reasonable that they be allowed to use sleep gas or tranquilizer, similar to what doctors use during operation. Seems better than killing them.
Downside could be lawsuites against the police for "damages" that the sleep gas caused.
As nice as it would be to have a reliable nonlethal alternative to firearms, there simply are no nonlethal weapons capable of stopping someone who is presenting an immediate threat. Tranquilizers and anesthesia are administered by an anesthesiologist, a doctor trained to determine a dosage based on a person's body mass. Too much of a dose, or even administering it too quickly can cause serious complications, or even death. Even under the controlled environment of an operating room 1/20 patients die under full anesthesia a year.
Sadly, the most reliable method of stopping a person presenting a threat is with a firearm.
Definitely questioning that 1/20 figure, I had full anesthesia with a surgery earlier this year and I'm pretty sure I'd be informed of that kind of risk.
...everyone's reliably incapacitated, and most people survive? (potentially everyone with adequate medical care which was not provided in that situation)
Downside could be lawsuites against the police for "damages" that the sleep gas caused.