they were casting the Curse in order to get back to the one person who could stop someone who was threatening their entire existence and the existence of every other person in the Enchanted Forest, including their unborn child.
So far on-screen, all we have seen is Zelena attacking Snow’s group and hijacking their castle. There is no mention that she is randomly killing peasants or anything like that (and why would she do that anyway?). Aurora told Snow that Zelena’s motive was her baby and not, say, to take over the entire realm. We have no evidence that Zelena is threatening the lives of anyone else (except for Aurora and Phillip but she left them alone until they betrayed her).
So what does a personal vendetta between Regina, the Charmings and Zelena have to do with the entire Enchanted Forest Realm, especially those who are not even part of the Charmings’ kingdom?
had Zelena not intervened and ‘mucked with the formula’, it appears as if the only price that would’ve been paid as a result of casting the Curse was Charming’s death, which is a significant price in and of itself.
What about everyone else who were unwillingly ripped from their lives in the EF and sent to a foreign land? Snow and Charming had no idea whether they could get back to the EF from Storybrooke. They are taking it on faith that they would defeat Zelena and find a way back, and while they have the right to risk themselves (and perhaps, their supporters), they had no right to make that decison for others.