Doubt

Jang Tae Su is a legendary criminal profiler in Korea. He was one of the first in his field and paved the way for criminal behavior analysts in the country. He has gained absolute respect and trust within the police organization. At home, he is a single parent and raises his daughter by himself. Jang Tae Su works on a murder case and realizes that his daughter is involved in the case. The discovery shakes everything in his professional beliefs and his relationship with his daughter. To protect his daughter, Jang Tae Su struggles to reveal the truth.

(reccing this as of episode 2 of Doubt airing)
Both feature an arc regarding a child killing a child now grown up, and the mystery of if they are capable of doing so again in adulthood. There is the push and pull of the perpetrator's family wanting to trust believe and understand them, but ultimately not being able to let go of that suspicion.
Both the killers have a stoic and unremorseful personality assisting in the struggle to accept them and their words.

Both are Crime Drama Thrillers with the Main Lead working in/ for police department (criminal profiler or a forensic expert), now living with a teenage daughter, who is forced to live with him after her parents' divorce n after her mom's death.

Father have strained relationship with daughter for abandoning her n her mom in her childhood

Daughter leaving her traces in the crime scenes of multiple murders.

Father suspects her to be a murderer and goes to every extreme to prove her innocence, ahead of the detectives investigating the cases, by conducting secret investigations on his own.