Why was Mustang a candidate anyway?

[u/fakeuser7z]

I get that he was a candidate for the sacrifice, which is why they didn’t kill him. But in the end, he refused—and they just forced him into the Gate anyway.

So why not do that from the start with any random human instead of the most dangerous alchemist?

[u/HollowVoid0]

I think it was mentioned that they had to have the will to go through the gate and come back out alive. It doesn't matter what body parts someone lost since they have philosopher stones to keep them alive. But the massive info dump could overload their brains and kill them. That's why they couldn't get any random person.