“We only shoot black people,” Georgia cop assures woman during traffic stop (vox.com)

I'm wondering, what is the police upset about, an admission of guilt?

This is clearly just a bad joke, but apparently a bad enough joke to get you fired. I don't know the full circumstances, but if the previous conduct of this policeman was good, and he didn't have a history of bad behavior, firing seems excessive. I am sympathetic to the level of stress the police is subject to, and sometimes humor is good coping mechanism. In this case it backfired.