Spoiler...
.
.
.
.
.
.
.
.
.
The lawyer, the reason he betrayed the detective is to help him to solve the case of her sister. There's a chapter where the detective hurt himself while investigating about her sister's death, and the lawyer saw that the detective bwas hurting just to find a clue so he decided to betrayed and investigate it by himself. I think there's no coincidence why the lawyer is connected to all suspected killers of the detective sister.
Mr. Liar