So you don't have a problem with a 60 year old man insulting a 19 year old kid playing a game?
I don't have a problem with fans in the stands saying things like "you suck" or "you piece of crap" to either professional or near professional athletes. Completely amateur level athletes are a different matter, but college basketball is well beyond say, little league baseball or pee wee hockey or even high school sports.
I would have a problem with a 60 year old man belittling a 19 year old in another venue, but that was not the case here.
He plays games on TV and may be rich for a few years some day. That makes him a dancing monkey for our amusement.
Marcus Smart is an adult who is going to be going toe to toe with the best basketball players on the globe in a few short months, along with all the physicality that entails, and on top of that, he will be expected to put up with the pressure that comes with being a franchise player for an entire city of basketball fans, manage a small fortune in salary, and deal with the criticisms he gets from fans and sports media alike on a day-in, day-out basis.
And every one of us would trade our own lives for his in the blink of an eye.
I don't feel bad because some 60 year old nobody yells at him. We fans get emotional and say things that constitute heckling. There is nothing wrong with that. If Marcus Smart is incapable of dealing with that, then there is a 0% chance he will handle everything else he has in front of him.
If this particular fan was doing other things besides saying "You piece of crap" then that is another matter.