However, this 'diabolus in musica' wouldn't have been played on the piano in the 1400's. There were no pianos yet. They may have had hurdy gurdys. Perhaps it was the lutists who were strumming chords with a flat fifthe until they were chastised severely. By the way, when talking about western music it is important to remember that what we're talking about is only western music, not all music.
Many things were condemned as belonging to the devil back in that day. Astronomy and astronomers fared far worse than music and musicians in the realms of blasphemy though this ocurred long before the middle ages and well into the Renaissance, the 1600's (which wasn't always the rosy era it's occasionally made out to be). After that, about 300 years later and again in the west, the devil stood to blame for the Blues, Jazz, and Rock 'n' Roll without much ado about astronomy or the Locrian mode.
Recent discussions about the Locrian mode involve whether or not it truly exists or if its importance only exists in theory, or to state that its tonic chord, the flat fifth, is unstable and seeks resolution, that it cannot exist without something else to build upon. From the devil to dismissal, for centuries, theoreticians of all kinds have tossed out the Locrian mode and occasionally music other than western along with it.
Taken in this light, the Locrian mode is extremely poetic. And, considered visually on the piano as described and pictured above, its dastardly and diminished flat fifth'ed tritone is in perfect balance. It has the white D key in the center, a black key to each of the D's sides and white keys to either side of each of the two black keys.