I want to collect the guard armor, all 9, for each hold. But, my current character isn't a murderer, and if he was, couldn't sneak-one-shot him to save his life. Plus, I have nothing in Pickpocketing. Is there a way to peacefully get guard armor?

not much help but you could get them into a fight-like start getting chased by a bear run up to a guard and hopefully hell start fighting it or if ur lucky dragons sometimes drop guard armour

You could try the barracks and see if you could grab some from there.

I tried the Whiterun barracks, but the only clothes lying around were boots.