whatever was 'brewing' makes little difference. the facts are: he left. just walked out. no matter what was happening at the time or the situation, that's a cowardly move.
i do understand what kind of pressure may have been on his shoulders, but, it doesn't matter much. morrissey was supposed to be a friend, they could've talked about it, alone, and worked something out.
i believe johnny was pressured by others, namely, his girlfriend/wife with statements like, 'you can do better on your own.' or 'you put up with that? i'd leave if I were you.'
johnny got caught up and fed up and made an emotional and rash decision. it happens, but it doesn't change the fact that what he did was horse shit. the move did help the Smiths jettison into indie legends, that, and morrissey's HUGE effort to stay relevant and interesting throughout his entire career.
morrissey was smiths (and still is), sweety, end of story.
without morrissey, the Smiths are nothing more than a mere spark in music history, morrissey has made sure they'll live forever, running on 16 cylinders.