I don't exactly hate the guy, but i am appalled by the way he treats other villagers, or should i say, "valuable customers". Mostly from the Special Orders quest, during which he cynically acts like a real life scammer, with "bio"/"organic" false labelled goods and/or exorbitant price. As well as the other things noted from other players, both above and elsewhere.
For me, it's mostly a "teach that guy a lesson" issue and overall "stability"/"balance" to keep said guy in check, or something...
Yes, he sells a wider variety than Joja, yes, his prices to the player at least are slightly cheaper than Joja (sunflower exception), yes, Morris is also not very like-able to the majority of the playerbase (if not downright way less), but NO! I won't have that guy have the monopoly. And besides, he still keeps sending letters with money even if you go Joja route, which means he still is making profit enough to throw money towards us that way, plus there is neither gameplay nor moral repercussion and NO! I don't feel bad for him, from going Joja!
P.S. Also, Joja is open all the way to 23:00, allowing for late hour shopping...