Fear is not a get out of jail free card. It takes a lot more than that to win even a valid self-defense case.
It is when you're a cop. You had Philando Castile in Minneapolis. Police shootings in Oklahoma and Ohio and the Carolinas and Georgia and Nevada. Coast to coast, north to south, police claim fear for their safety and the safety of others and they walk. Happens over and over again.
I think the standard is usually “reasonable” fear. It’s sort of vague, and subjective. That’s why so many beat the charges; it’s difficult to get twelve people to agree on the definition.