What Haewon did in the past was wrong: planning a wedding while simultaneously maintaining a relationship with Suha. It was clear from the start that once a relationship is bound by marriage, priorities shift. Haewon would inevitably focus on his immediate family, especially after having a child.
Haewon himself admitted to feeling comfortable with that woman, and naturally, one tends to gravitate toward where they feel most at peace. He even claimed to have given up his comfortable life for Suha therefore it’s highly likely he would eventually fall for his wife and neglect Suha entirely.
Being much older Haewon should have known the consequences of starting something with Suha, yet he played the fool when faced with reality. Given Suha’s mental state and background, his reaction was predictable, and Haewon was the one who took that risk. Suha was neglected by his mother, raised by a father who became abusive due to mental instability, and ultimately witnessed his father’s suicide—spending a week with the decaying corpse. Haewon should have known better regarding the long-term psychological impact this would have on Suha.
I often encounter this trope—marriage for the sake of parents or status—but it’s rarely executed well. Suha shouldn’t feel pity for Haewon’s decision to marry, because Haewon knew all along that pursuing a relationship with Suha under those circumstances was a mistake.