BK, they want to save Serenity. You don't seem to be giving them that chance, saying that logically that Jack will kill her. They're saying if that happens, there's nothing stopping them from releasing their rage and going after Jack. They want to save Serenity, the problem is that they need [b]YOUR[/b] approval. You speak of how much you don't want to take away someone's character, but you don't seem to want to open up any chances that she could be saved. In a world with aliens, magic, and technology beyond our understanding, logic and limitations are fairly hazy. 

They have ways and ideas to save her, they just need your approval. They're coming to you with what they want to do now, instead of doing it behind your back or without your permission. Once again, the actions of what others do is reliant on what you tell them.