I don't think that she got crazy...she just finally realized that she has so much power and that she couldn't do anything with it. Tara's death brought out the fact that she should couldn't do anything to stop Tara's dying because it would be an abuse of that power. She just asked herself what's the use of power when you cannot use it. Might doesn't make right. What was it that Anakin said? "One day I'll be the most powerful Jedi ever. So powerful that I can stop people from dying." And we all know what happened then. Once Willow gave into her magics she thought that she was a god, and that simple notions of right/wrong were irrelevant.
As for Oz and Anya--I wasn't speaking literally but figuratively. Oz doesn't like the fact that he is a werewolf mainly because he could hurt someone he loves, and Anya realized especially with this episode just because she is a powerful demon, she could do something that she regrets--like hurt Xander whom she is still in love with.
The same can be said with Angel and Spike. Here you have two demons who were vicious in the days' past(and they reveled in it)but once they got their souls back, they realized what they did and they were not happen campers. I mean they tortured and killed hundreds just for folly, and they are just starting to atone what they did.
As for Giles and Faith, these are people who are also in touch with their dark-sides and try to hide it as best they can because they don't like the things that they do when they are that way. Buffy herself even denies the fact that she has a darkside--the power of the first slayer. Without Giles and the others keeping her in check, she would be nothing more than a killing machine.