The main thing many people have missed is that the characters Hee-joo and Hyeon-seong were ready to move on from the incident. Hyeon-seong was aware that he got cheated on, and yet he knew his wife regretted it and chose to move on. In the last scene where Woo-jae tells Hee-joo her husband was the one who crashed into him in the car incident, she tells him that it's not true, which felt like she said it to protect him. These two characters were protecting each other, after all, they raised children and they were a married couple who were together for many years, no matter what happened.
Woo-jae was only a puppet in this story, who was selfish enough to not realize the consequences of the affair, even if it continued. Before he died, he told Hee-joo that her husband was the one who crashed into him, and when the answer he got didn't satisfy him he got furious. He wanted to have a relationship with Hee-joo, as he said so before as well. His tries didn't end up how he wanted it to, like the time when Hee-joo acted for the car camera and showed the recording to stop working together. This led him to be so furious that he wanted to kill Hee-joo because he wasn't satisfied. Like all the characters, he had very bad actions. Considering that he figured out Hyeon-seong wanted to kill him during the car incident when he crashed into him, it's not unlikely that he wanted to get some form of revenge. Regardless, it seemed like he did all of this just to be together with Hee-joo.
No matter what happened between Hee-joo and Woo-jae, Hyeon-seong still wanted to move on. It doesn't matter what choice he makes in the end, as he can decide to continue the relationship with Hee-joo and no one can say anything about it. Hae-won wanted revenge, so it started bothering her when she saw that their relationship wasn't falling apart, so she had to ruin it. It didn't seem like she caused most of what happened, as it was somehow going to happen anyway, but it could have turned out to be better in the end had she not interfered. She was the cause of many of this. Maybe she did believe that Woo-jae was going to change at first, but then as time went on, she realized that he didn't. So she gave up on him and wasn't too upset when he died as well. Sure, she did seem to get a better ending, but it doesn't seem like she can live a happy life anymore, and she sacrificed her happiness because she wanted to get revenge.
I do think that it would have been better if she had given up midway through and watched how Woo-jae acted if he hadn't died, and how Hee-joo and Hyeon-seong were going to deal with it. But Woo-jae was the puppet of the story anyway. Hee-joo did everything to protect her family after cheating, even if she wasn't the best mom or she had bad actions like everyone else. Great acting though.
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