In the second before Angel cruelly snaps his neck, Giles makes his peace.
He has always put the world first. He would have killed Dawn, he smothered Ben in order to take down Glory, has tortured demons for information.
He has always being willing to do the tasks his slayer refused. She still possess an innocence he lost long ago. He knows that Buffy will never be able to stop Angel, even knowing the stakes. He will force her hand using the only means available.
Once again, he will help save the world. It's what a watcher does, after all.