I am very happy that police will often choose not to use lethal force, even if it would be allowed. The point is, however, that had such a thing happened without cameras, proving it (versus an officer's word, which is often treated as Truth in courts) is very hard.

Cameras' purpose is to allow gathering evidence in the (hopefully rare) instances of inexcusable behavior by officers, and to encourage them to do things "by the book" rather than stretching the truth (lying) about what actually happened.