Ah, when it comes to how these folk think, you're decent aren't you?. They're a different kind of people from us.
This is a response to being asked whether a plan will work. I'm confused about the おまえ、まともじゃない part.
The people in the discussion pride themselves on being decent, so translating おまえ、まともじゃない as "you are not decent" would not make sense. It would make sense if おまえ could mean 'they' but I doubt that is the case.
My best guess is that じゃない is rhetorical, and that the phrase means "you're decent aren't you?". Of course I could be way off.
If I'm right, it got me wondering. Would a fluent speaker instantly see this as rhetorical, or would it trip them up and require them to re-read the passage a couple of times like I had to?