Alright, the Huge Materia.

I think that the party was partially motivated by revenge, at least some of the time. Like they felt they had to oppose Shinra, because it's what they do. Cait Sith implied this after the Huge Materia quest, when he said, "Haven't we troubled them enough for now?" or words to that effect. I think that the plan to take the Materia might've been a pretty rash decision at first, it's just fortunate that circumstances provided some justification for it.