I just don't get why people can't forgive Xander for the choices he made. People make excuses for the other characters all the time ("Well, Buffy had killed Angel; she needed time away" or "But Willow was an addict, she had a problem"). Can't people recognize that as hard as it was for them, it was just as hard on Xander but in a different way. He had other stuff going on. Here he was, average Joe, surrounded by all these extraordinary people. We see by his speech in "Potential" that this deeply affected him; I know it had to be majorly frustrating at times to have such a big heart and all you want to do is help but you can't. You can't physically 'cause you're not strong enough and you can't magically 'cause you don't have the power. And then when friends are leaving and going all crazy, who wouldn't get a little touchy? Xander used sarcasm as his defense 'cause that's all he had. He couldn't fight Buffy and he knew it, so he used his words to try to get at her, the same way a LOT of people do when they're pissed off. It doesn't mean we should never forgive them because in the heat of the moment everyone says things they don't mean totally. Which is why I'm able to look past his assholeishness in "Becoming" and "Dead Man's Party".
In "Becoming", Jenny's death at the hands of Angelus, along with Angelus' stalking of Willow, Buffy, and the others was still fresh in Xander's mind. He had seen first hand the type of things Angelus was capable of, and it terrified him, which was why he was so apalled at the idea of wanting to try to restore his soul in the middle of all the apocolypty stuff. Lying about Willow doing the spell, in my opinion, was smart because I agree with AndrewCrossett that Buffy more than likely would have been holding back the entire fight and it could have meant death for her. She didn't need the distraction. Plus, as others have said, it would have been too late anyway because the spell had already been started.
In "Dead Man's Party", Xander had spent an entire summer sick with worry about where his friend was and what had happened to her, and having to step up and take over her job without being qualified to do it. And he had had to see Joyce sit by day after day worrying senselessly about her daughter and torturing herself over her last words before Buffy left. Put yourself in his shoes, and wouldn't you be pissed off if she showed back up one day, and then all of a sudden acted like she wanted to leave again? You'd have a WTF moment, too, I'm sure.
As far as leaving Anya at the altar, can you not have pity for the man who grew up in an abusive home and was terrified that he was going to end up like his drunken, mean, abusive father? He saw what his life was like, and thanks to the demon, his own fears were amplified. People seem to forget that he left Anya to spare her from a life of hardships, because at that point he didn't believe in himself to know it could be different. It's sad for both of them, not just Anya.
I guess I just can't understand why some people can watch the show, knowing the background of all these characters, and not understand more that all of them have reasons for behaving the way they do, and sometimes it might not be good, but why can you forgive some- some who have KILLED- and not others, when their "crimes" were far less disastrous?