"We only kill black people." says Georgia policeman...

Sure. So what?

This cop has no doubt arrested people for assault. Say one of those people went to court and his case was "Well you know your honour, assault isn't as bad as murder. There's way worse criminals than me out there!"

That would be a perfectly reasonable thing to say. It would be a perfectly true thing to say. It would also be entirely irrelevant.

The cop did a "bad" thing. "Other things are even more bad" is true, but so what? It literally has nothing whatever to do with this incident.