Yoo Seung-ho, what kind of incident happened in the past? (Chaebol X Detective 2)

Park Chan-geon (Lee Do-yeop) expressed his pity to Joo Hye-ra, saying, "You're still chasing Yoo Seong-won, aren't you? That's why you came back here, right? I understand how you feel. That it's a case you can never forget. But you confirmed Yoo Seong-won's alibi back then, didn't you? That's an obsession. Don't you know that's what ruins a detective's career?" However, Joo Hye-ra replied, "It's not an obsession. Yoo Seong-won kills people again. I will stop him." I'm so curious to see how this unfolds!