I agree. Xander had gone to Buffy with the sole purpose of telling her Willow's message. When he started with "Willow sent me to tell you..." Buffy cut him off with a hopeful tone that made him pause and reconsider, and then decided to go with "... kick his ass."Xander's "Kick his ass" line is blamed on jealousy and hatred of Angel, but I honestly think that's not what it was. Xander had no idea about Angel's connection to Acathla. All he knew was that Buffy, by her own admission, was unsure of whether she could bring herself to kill Angelus. As it was, she struggled to overpower him- if she held back, Angelus would have certainly killed her. Whether he had the right to keep this from her is another matter, but I think that his motivations were to protect her and save the world.
If we look at the situation clearly: Willow wasn't well enough to perform such a powerful spell, and Buffy had proved time and time again that she was not ready to kill Angel. Xander was most probably looking at it from this angle. He didn't have faith in either Willow or Buffy, and did what he thought was right.
I find it silly to assume he did it out of jealousy. If jealousy was all he needed to kill Angel, he would have agreed with Kendra when she demanded Angel should be killed, but he didn't agree with her. Ever since he lost his soul, Angel had done horrible things to the gang: killing Miss Calendar, attempting to kill Xander and Willow, kidnapping and torturing Giles, terrorizing Buffy, and now he was about to end the world. I can understand Xander getting scared of the outcome.
Though I wish the whole lie thing was solved in S3 with Buffy and Willow learning the truth then.