BtVS Episode, "Becoming, Part 2" AU Question: If Xander Harris had told the Slayer known as Buffy Summers about their other friend's plans on restoring Angelus' soul, what would result in the boy telling her what she wanted and needed to know?
(I love how you’ve framed this question, btw)
First of all, I think Xander delivering Willow’s message wouldn’t have necessarily translated into a different ending. Buffy might have had to kill Angel/us even knowing that Willow was trying to restore his soul.
What I do think is that Buffy would’ve entered the battle in a different frame of mind. Some people argue that Xander wasn’t trying to be a jerk but he was actually trying to give Buffy the strength she needed to fight Angelus/her former boyfriend. I strongly disagree. I think Xander 100% chose not to deliver that message for purely selfish reasons, reasons that I sort of get. Angelus had killed Jenny and had just kidnapped and tortured Giles, so I understand why Xander wasn’t really rooting for Willow to succeed in performing the spell. Plus, Willow had tried once already and failed, so maybe Xander considered it didn’t really matter whether Buffy knew about it or not if it wasn’t going to work anyway. Regardless of whether his motives were valid or not, he ended up stripping Buffy away of a vital piece of information that might have made her enter the fight with a different mindset.
We need to remember that Buffy has always found power in her emotions. And not in any kind of emotions; what drives her most of the times is her deep love for others. Now, to go into this fight, Buffy had to let go of her love of Angel because she needed to kill him. Xander’s message reinforced this mindset. She needed to forget she was fighting her former boyfriend and view him only as a target. It’s also important to remember that when Buffy arrived at Angelus’ crypt, he still hadn’t completed the ritual to awaken Acathla… What I’m trying to get at is that had Buffy known the possibility existed of Willow restoring Angel’s soul, she might have approached her opponent in a different way. Her emotions might have helped her stall Angelus or to drive him away from Acathla until the spell had been cast. She might have spent less time fighting his minions so that she could focus on getting him away from the statue, you know?
EDIT: Plus, she would’ve been better emotionally equipped in the event of Angel getting his soul back mid-fight, as happened… Not knowing that Angel getting his soul back was actually something that could happen as they were fighting made everything so much more traumatic for Buffy, and that’s all on Xander.
Of course, this is all highly hypothetical. There’s really no way to know, and there’s a big chance she might have ended up having to kill Angel/us all the same. But… (and this is, I think, the biggest way in which things might have turned out differently…)
Xander’s “kick his ass” had an underlying meaning, too. What Buffy got from that message is that neither Xander nor Willow cared about how difficult it was for her to be forced to fight and kill the person she loved. After Angel’s soul was restored and she had killed him, Buffy felt she couldn’t go to her friends for support and comfort because they wanted him dead and they cheered her on to kill him. How could they understand her pain?
I think that had Xander delivered Willow’s true message, Buffy might have tried to reach out to the Scoobies after the battle because she would’ve understood her friends had her back and weren’t actively rooting for her to be forced to murder the man she loved. The fact that Willow was trying to restore Angel’s soul meant that she didn’t want Buffy to be in a position in which she had to kill the love of her life with her own hands, and more importantly, she wanted Buffy to have the love of her life back.
Lacking the knowledge of Willow’s true message, Buffy found herself in a position in which she had no home, no place at school, and no sympathy from her friends regarding what she had gone through with Angel, and I think the last item on that list made all the difference. She truly felt there was no reason for her to stick around.
Ultimately, even if knowing this hadn’t really changed anything in the slightest, Buffy was still entitled to know.
TL;DR: forever bitter at Xander for not delivering Willow’s true message