I don't think any gem truly believes in their cause. They just follow orders, even the Diamonds. Even White.
And without Pink, that was really all Yellow had left. Order! Purpose! To distract her from the gaping, sucking wound where her heart had been.
And that was why they were on the one hand so implacable - they had nothing to affect them, because they believed in nothing.
But when their 'heart' came back and told them that what they were doing was wrong - how could they possibly resist?
Even a Gem would realize that mindlessly reproducing is pointless for an ageless species. They do it because it's what they were ordered to do.
That is why I hope that season 6 will give us some answers.
Notice White Diamond's reaction here: https://www.youtube.com/watch?v=kZFYXKSCSa4
She says "I am supposed" FIVE TIMES. "I am supposed to make everything better".
Supposed... by whom?
Her creators, probably, but who are they? And why did they create her? Is she a terraforming, self-replicating robot - a Von Neumann machine, as it were?