I can certainly understand why the guy slapped the kid, it really acted like a brat. And pain through corporal punishment is certainly an effective way to get kids to do or not do certain things. But it's still a bad lesson, it only teaches that you can do anything and demand anything as long as you have power.
That kid clearly had anger issues, pretty obviously because his parents neglect his parenting. The kid is out of control, and hasn't learned ways to deal with frustration. What the kid doesn't need is some wake up call, it needs to learn that it cannot have everything it wants, and best from someone who is firm and demands certain behavior, in a loving and caring. E.g. that he can join the game later again when it's his turn again if he puts down the bat right now.
I thought the kid deserved the slap, his own parents weren't teaching him or punishing him in any way so, Hector's cousin stepped in. I know that my dad would have talked to the parents when the kid started pulling the flowers and maybe even took him upstairs into the bathroom for a spanking right then, even if it wasn't his kid. This would have also been almost 30 years ago but the kid deserved it.
The parents should have raised their child with better manners and more discipline. That does not however mean slaps or spankings. I have four kids ranging from three to fifteen. They have never been spanked or hit, and I have gotten many compliments about their good manners. It is possible to be firm and not spoil a child, without striking them.