"Not much. Angeal's going to desert, too, as soon as the mission's over." Technically before, he supposes, but it doesn't make much difference. Sephiroth might not be able to course correct anything before that, given how complicated escaping such a coordinated mission midway through would be. Especially for someone so important. (So closely monitored.)

Sephiroth's warning does give him pause once more, but it's briefer, again, than before. He doesn't have to weigh his conscience much, on this one, given what he knows. What he's already done, himself, years too late. "If stopping him means saving everyone else, then... I guess it's no worse than what he'll do to himself, if you don't."

Either way, it ends in his death. By Sephiroth's hand, by Zack's, or simply by degradation.