The backstory of Harry is a cheap way to ......
balance out the outrage. While I thought Harry shaking the kid was fine, slapping the kid that hard was out of line. A quick light firm slap on the lower body would have been tolerable response to a kick in the shins by the kid. Sometimes a kid should know a reaction has an equal and opposite reaction. But if I found myself slapping the kid that hard mistakenly(sometimes the action outweighs the off the cuff intention), I would have apologized sooner than Harry did. But other than that, I actually found him to be one of the more tolerable characters on the show until..... they showed the episode where he is depicted as the guy who broke his wife's jaw. Way to make it easy for the stupid women to justify their overreaction towards Harry though at least in Aisha's case, she is shown as not seeing a correlation between the two incidents. Still, she clearly eggs on her friend regarding the slap for most of the series instead of taking the rightful stand of not caving in when Harry hit his wife.
They do show Harry losing his temper once in one of the early episodes. That would have been enough to put enough grayness in his other positive features instead of making an outright wife beater.