There's no objective measure of decency. I'm qualified to offer my subjective input while evaluating the candidate because I'll be on the team that has to work with them.
And frankly, I'll happily take the L if the candidate rejects me as the interviewer at another position. I have no right to assert that I'm the best possible candidate for their role because the job at hand isn't strictly about quantitatively measured qualifications. Though, if I rejected a candidate for being unpleasant, chances are I'll withdraw my application upon seeing they're and interviewer. Why would I change my mind about working with that person?