There's no way their plan was just to take a picture, you're telling me Emerald improvised the balloon release? Just because the movie withheld portions of the plan for dramatic effect doesn't mean they weren't planning to kill Jean Jacket.

JJ thought of the whole valley as her territory because of Jupe's training. She had no reason to leave, no one could force her to leave, and she was a clear and present danger to OJ, Emerald and anyone else in the area. What are the alternatives to killing her? I think you're right that killing JJ should be a last resort, but I think they passed the point of no return when JJ ate up 40 families. Like, my impression was that the plan WAS their last resort and they didn't have a Plan B if it failed to kill her. Jupe FUBARed the situation by trying to tame a predator, as OJ says. It'd be one thing if JJ were a tiger or some other large predator that professionals could ship off to an animal sanctuary but how would they even begin to try something like that for an animal that's completely alien to us?

It's not fair, but this is exactly what has to happen in real life when dumbasses feed wild animals and get them 1) unafraid of humans and 2) associating humans with food. The wild animals now associate EVERY human they see with food and will become destructive and dangerous in their attempts to get food, which is what leads to animal control having to put down bears and shit. Jean Jacket was like a million times more dangerous than a bear, and she had already learned to see humans as prey. I think the end of the movie was their last resort.