What I would say about this is that we just don't know enough about what Taeyong did to say whether or not he was a real piece of work.
Like there's a big difference with you coming up to me and me saying "get lost moron" and then walking away as that's just me being a dick than me saying something like "hey look I found your iq, oh wait there's nothing there? Just like in your head moron" every time I see you which I am going out of my way to constantly make you feel like you're dumb.
All we really know is that Taeyong called someone fat and while the OP did use the word harassing which would fall more into bullying however I feel like there needs to be more of an explanation if it is the 2nd one.
Also for the he was a kid point, there are kids which have ended up killing other kids, bullying whether it is an adult doing it or a kid is something you should curb the persons behaviour to stop them if you are in that position of power and it's also easier for kids to learn than adults so makes even more sense to do it then.