You also forgot that Faith shot Angle with a poison arrow that needed the blood of a slayer to cure. That was the reason why Buffy confronted Faith so she can drag her to Angle so he can bite and drain her blood until she was dead. True, Faith was not that good to the males of the series. True, the relationship between Buffy and Faith got more toxic as both women were not willing to be equal to each other. Yes, she killed a white male she thought was not a human. It was a very lame killing. Faith accepted it as a accident as a part of the way she deals with vamps and demons. Buffy on the other hand, wanted a good long girls only heart to heart talk about the death. When Faith rejected that social bonding, their friendship became worse.
If I was a slayer, and by accident I killed a real man -- I would have done the same as Faith. There is always civilians killed in conflicts with two parties in a conflict. How she dealt with the death was in my judgment the only best result. True, she could have called the police and confessed the the death. Got arrested, pleaded guilty and gone to a woman's prison. But, since she is a slayer, a vamp or a demon could have killed countless humans during her decades in prison. Only after her death, would another slayer would have called to confront and kill vamps and demons. Or, dealing with her guilt, she could have asked Buffy to kill her so another slayer could be called. So, there was suicide to take care of her guilt -- something that is irrational and would not have been accepted with the fans. She could have gone to prison for decades, and left Buffy alone to kill vamps and demons alone. And if Buffy was killed, there would not have been a active slayer to kill vamps and demons. So, there was only one rational answer to the crime -- cover up the crime and move on. Something that Buffy and the scoobies were willing to accept. Faith accepting the death was not a ideal but it was rational. I am not a idealist, but I am a rationalist. And time and time again, idealism classes with rationalism.