I don't think there's a great answer here. Without special legal protection, you're asking police to take a job that would require them to make decisions in a couple of seconds and if they make the wrong decision, they face a high risk of either death (should've shot but didn't) or life in prison (shot but shouldn't have). I wouldn't take that job if it payed 500k/yr, but we're asking officers to take it for 60k/yr?

I'm not asking for police to be protected if they're malicious, but if a police officer completes all his training correctly and tries his hardest to make the right decision in that pivotal moment, but makes a mistake and shoots when he shouldn't, then he's sentenced to life in prison if he is held to the same standard as a civilian.

And people wonder why their police officers aren't the best and the brightest: anyone making a rational decision under these terms wouldn't take the job.