Nana

The parents knowing their kid is likely psychopathic and not caring for him properly, but instead letting Seo-In be harassed by him is infuriating.
I actually really like how Seo-In has been portrayed from the start, a very complex personality and character, moulded by an onslaught of confusing and hurtful experiences since he was small.
His problematic behaviour is so entertaining to watch and how Yeongwoon's role is that of a catalyst whether it's for short-lived solace, falling more deeply into despair and harmful patterns, or potentially letting some things go and actually feeling he's worthy of happiness and security.
A delectable story of toxic themes!

I find this to be a realistically painful cheating story and actually not entirely unhealthy. It's worth a read to think about the topic, if you're not particularly triggered by it.
I personally feel bad for all 3 people involved.

Ofc the healthiest and fairest thing to do would've been for Woohyun to be honest with Seok Il from the start, since he felt regrets and wanted to pursue things with his ex AND he was unsure/wary about his existing relationship (he kept saying how he felt he "settled for it"). Not as a free pass to fuck around, but an honest break with a time to re-visit their relationship again, IF they both agreed because they didn't want to give up their relationship altogether yet and they set certain conditions. It's not a pain-free solution, but it's much better than breaking your partner's trust and taking x100 times the effort to rebuild it, if it's even possible. Or they could've broken up right away, and even if it was the wrong choice for them, at least there wouldn't by any lying involved and down the line things could still change.

I do think Woohyun regretted it, he didn't wake up and decide to be hurtful to the person he held close and loved for 6 years. It just wasn't explored enough in the end of the story, because the initial reaction he had was to quickly try to make up. Which Seok Il also subconsciously did too (despite being terribly hurt), afraid to lose him again, so they avoided going deeper into what happened, they just needed that fast reassurance that they got back together. It'd been great to see their actual healing process from all that happened.

As for Minhyuk, he obviously had very deep regrets and alot of unresolved feelings for Woohyun and what happened between them. I understand how he'd want to make up for everything and get a chance to live what he'd been deprived of bc of his family's situation. But it still was a shitty, immature way to go about it. Missing the right timing can really suck, especially when it wasn't up to you, but you're still responsible for how your actions affect others. He could've taken the opportunity to talk about that, give Woohyun the chance to consider it and if his feelings were not reciprocated, gradually move on. Or if they were, wait until they were broken up before taking things further.

Unfortunately, cheating most often comes from a place of hurt and insecurity and it only passes on even worse feelings to others when it's done. It's not always the end of a relationship if it's not done habitually, but to recover from it takes alot of will, time and work. Not lying - even if you're unsure of how you feel exactly - and discussing/thinking things over is the best thing we can all do to prevent causing more harm in complicated situations like this.