How interesting, I hated (viciously hated) Xander's "kick his ass" lie back when it first aired. But on re-watches all I can say is that it was the right thing to do because I firmly believe (as I suppose Xander did) that knowing Willow was working to re-ensoul Angel would have made Buffy slower if anything. She would have been hoping for Angel's return, she would have hesitated, she would have pulled back in order not to have to kill him. Knowing she had to kill Angelus, to me, and being without hope of Angel's return is what saved her life, because she was faster and better because of it.