The episodes should've been an hour long, having them range from 30-45 minutes hurt the show. If they made the episodes longer they would have had time to show how Se Eun was being bullied, by who, and for how long she was being socially isolated from everyone. Showing how each character was hurting her(intentionally or not) would've made the parent's motivation make a little more sense, i.e. a character did't actively bully her but didn't talk to her and maybe ignored her to save themselves from also being bullied.
That being said from what i understand there was only ONE bullying episode and i understand it was bad but to me it wasn't believable that she killed herself over that one thing, it would've made more sense if there were multiple things happening and that was the tipping point, maybe even her best friend stopped being friends with her. It also seemed like she killed herself right after she found out which again didn't make it believable, if after that incident a month passed and nothing got better and her best friend wasn't talking to her then it would make sense.
There are a lot of unanswered questions as well involving character backstories( the swimming pool PTSD for one thing) which were never touched on and the "ghost" concept which seemed to have been forgotten(sadly, because i really liked it). There is going to be a second season so maybe they'll explain even in passing but I don't know.
The very last thing at the end why are the mafia allowed to kill more than one person? if that was the case why didn't one of the characters try and kill everyone to end the game? and why put the representation of your dead daughter with everyone? why not leave her out?