I believe if you assault some one, you might try again, and then it is battery.
It isn't battery until it is battery; but it never was battery and it wasn't a public safety issue in need of firepower. It was a power and ego trip by the retired police officer on an unarmed citizen who unintentionally annoyed him, which he took gravely disproportionate offense to -- like an adolescent gangbanger who has been "dissed", so he shoots someone.
"Might try again"? And I might win the lottery! The victim never got the chance to try again, nor to raise his little daughter or have another wedding anniversary or even see the movie. He never even stood up. I am generally favorable towards professional police officers; but please -- you are defending the indefensible.