Sorry but i assume you haven't finished the story yet according to the comment you replied in other comment session on the same day. You said that you read till ch.41 so i figure why you think of Taeyung like this and it seem you hate him so much. I get it but i would like you to finished the story first. After that you might understand why many people is sad about his dead or I really am happy to point it out for you from my perspective
First, why I am sad about his dead is because HE WAS MURDERED and the cause of murder is him being gay. That's tragic and sad in itself. Unfortunately, in real life something like this is actually happens. Heesoo parents causes it so that his son will be purify from gay demon a.k.a Taeyung as cult leader claimed. Taeyung died because of the cult idiot and greedy jerk bastard. HE DID NOT DESERVE THIS. Anyone who read the story would think it's sad and tragic that he has to die like this.
However, i get where you are from and i am so sorry that you have to experience something like this, but the story of Taeyung isn't like that. HE IS NOT PHYSICAL ABUSER. The story reveals later that he acting strange after the cult leader lead Kitae ghost to haunted him and Heesoo said it himself that Taeyung is acting weird and spacing out a lot. Even Heesoo blamed himself for neglecting Taeyung. Heesoo just blushed it off as Taeyung starting to change and his love might die down too but who in the world would have thought it’s because of the ghost. The only time Taeyung is violent is when they fight before Taeyung went to business trip and died because of Kitae ghost. Taeyung was possessed. He didn't do that out of his own will even Heesoo said Taeyung seems like he isn’t in his right mind. That explained the nature of Taeyung in Heesoo perspective. Heesoo even gets mad every time Yihyun talks bad about Taeyung especially when topic of “hitting” comes up. Until later they found out why Taeyung did that. It’s bitter. Anyway, it is no one’s fault EXCEPT THAT MF KITAE. Taeyung’s intention is always to protect Heesoo even when he dies, only regret he has is that he cannot protect Heesoo anymore. That’s why even when he is a ghost. He tried to stop Heesoo from killing himself and protect him from that mf Kitae.
Yes, Taeyung was only physically violent after becoming possessed, however one can argue that the emotional abuse he inflicted started at a much earlier age. Sure, his motives were to protect Heesoo, but he had zero faith in him. Instead of actually communicating and working together with Heesoo he just decided for him. A relationship where one person makes all the decisions isn't a healthy one, regardless of the situation. It was also mentioned throughout the series that he was very jealous and that scared Heesoo.
Hihyun on the other hand has faith in Heesoo which actually builds up his self confidence rather than just putting shackles on him. And it's because of his faith and support that Heesoo is able to tell Taeyung that he wants to live and be happy again.
I wouldn't go so far as to say that Taeyung's only regret was not protecting Heesoo. In some ways he was the catalyst for everything. Because if he provoked Kitae, then he might not have held such a grudge against Heesoo. Although he protected Heesoo he was also dragging him down for his own selfish reasons.
All in all, yes his death was tragic but we shouldn't gloss over all his past actions just because some of them was due to the possession.

I don't get why many people here are sad about taeyung's death.
Yes he did try to protect heesung from his parents and that pervert, but.... isn't that just the bare minimum to do for someone you love? It's the least that one should do.
And after reading it, I noticed that taeyung wasn't really the ideal partner. From heesung's memories; He was overly controlling, he always forced heesung to be more adventurous and outdoorsy even though he is an introvert and took advantage of the fact that he is a pushover. Never allowed heesung to have his own friends or talk to strangers. Only let him be friends with his friends(even one of his friends casually mentions that "taeyung gave heesung a hard time when he was alive and now he's doing the same even though he's dead) and for the worst part, the day before he died, teayung hit heesung.
Maybe it's cause I come from an abusive/strict family, but I just don't agree to abusing the ones WE LOVE. Strangers? Sure, who cares about hitting a stranger? We all get into fights and it's totally okay.
But I can't fathom the thought of hitting the one you love. No matter who they are. Siblings, cousins, friends, lovers(only the loyal ones, fuck them cheaters) anyone... Just the thought of hitting someone you love immediately destroys that relationship. Hitting someone once and then getting away with it creates and endless cycle of an abusive relationship. I don't support that. No.