I have always seen Buffy's views on killing humans to be that she doesn't ever want to take a human life, but if it's necessary to protect those that she loves and the world, she will. The cases you mentioned with the zookeeper, the coach, and Warren were all accidents, so I don't think it's fair at all to say Buffy had no issue with killing them. And maybe we don't see Buffy grieving her decisions about those men, but mostly it's because there's not time for her to grieve or there's a cut scene to a different day or something. So just because we don't see Buffy feeling badly for what she did in those cases doesn't mean she didn't.
The only time I ever thought Buffy's ideology about killing humans was screwed up was when she attempted to kill Faith to feed her to Angel. I understand her vision was clouded by her anger and desperation, but I never agreed with her reasoning for attempting to murder Faith.