would've been better without the slapping...
At that point, the character of Fletcher becomes undeniably abusive and irredeemable. If the film just showed him pushing the students really hard while never once laying a finger on them, it would've made for a far more complex and morally grey film. If they didn't show him physically abusing Andrew or purposely setting him up to fail at the end, there could've been stronger debates on both sides as to whether Fletcher was an asshole or not.
As is, Fletcher is undeniably a villain, which takes away much of the ambiguity Chazelle intended.
Who's with me?