But see, now we are already specifying what beliefs/themes the fantasy would need to be based on. In most moral systems, wisdom / proportionate means are considered a part of goodness.
Your argument only makes sense if you agree with moral relativism. Even then, I think it is safe to presume that creatures from a realm based on goodness have a better idea of what goodness is than your leader from a morally gray realm. Furthermore, it would be intuitive to any good character to trust them for that reason.
In reality, good people may not agree on things, but they generally don't start killing each other the moment they disagree, especially if they only recently met. It really is pretty convoluted to even try to come up with something, and if you try to force a fit that doesn't work, it will show in the quality of the story.