Warning: spoilers for Dead to Me season 2 are ahead.
It may seem like Detective Ana Perez (Diana Maria Riva) is the antagonist on Dead to Me, because she knows that Judy (Linda Cardellini) responsible for a hit-and-run, Perez is also getting closer to learning that Jen (Christina Applegate) killed someone too and that Judy helped her cover it up. Through seasons 1 and 2, Judy and Jen were always worried that Perez was going to catch them and put them in jail. But when push came to shove, the detective wasn't the antagonist at all. Perez even let Jen go on Dead to Me after Jen confessed to murder.
At the end of Dead to Me season 1, Jen killed Judy's ex Steve (James Marsden) in part because she was scared of him and in part because she was angry at what he was saying to her. Steve showed up to Jen's house furious, looking for Judy, but Jen was alone with just her sons in the house, sleeping. Steve then started in on Jen, blaming her for driving her husband Ted out of the house and claiming that Ted had intentionally jumped in front of the car that hit and killed him. In a blind rage, Jen hit Steve in the head and he died.
At first, Jen attempted to cover up the crime so she wouldn't have to go to jail and risk her children losing another parent. But when she learned that the police suspected that her son Charlie (Sam McCarthy) may have killed Steve, she decided to turn herself in to Perez. She showed up at Perez's house in the middle of the night to confess. Jen even drove out with Perez to where she'd buried Steve's body, but they weren't able to find it in the dense forest. For once in this series, Jen didn't lie about anything. She explained that her attack hadn't been in self-defense, that she'd just been angry. But she also mentioned the truth that she had asked Steve to leave, that he hadn't, and that she was fearful for her children. But then, Perez let her go.
On the drive to the police station for Jen to submit a statement, the two began talking about their own mothers. Jen lost hers to cancer when she was 19. Perez's mother was murdered by Perez's stepfather in an act of domestic violence. The detective's experience with that tragedy and domestic abuse likely played a big role in why Perez let Jen go.
Previously in the season, Perez learned that Steve had left Judy several violent voicemails before his death. In all of them, he was screaming at Judy, telling her that he would kill her. At the time, viewers didn't know about Perez's backstory. But in hindsight, as she listened to Steve's threats, the detective looked more than just upset. She was even able to look past her hatred of Judy for a moment to ask if Judy felt she was in danger. Now we know why.
So with that in mind, Perez knows that while Steve may have been turning to go when Jen hit him, just before that he'd threatened Judy, refused to leave Jen's property, and made both women fear for their lives and Jen's children's lives. Perez wouldn't respect anyone who made other people feel that way, and she likely wouldn't want to see Jen's family ripped apart because of it.
She also had a much bigger fish to fry: Conveniently, other evidence pointed to the Greek Mafia that Steve was involved with being responsible for his disappearance. Since she and Nick were working that case in secret, hoping to take their sexist, racist, and fully corrupt police chief down, it was easy to lump Steve in with the takedown.
It also helped she and Jen were unable to find Steve's body in the woods, that Jen had done the right thing, and that Perez' ex girlfriend Michelle (Natalie Morales) left her for never being "able to turn her cop side off." It was a perfect storm. So when Perez says she wants to be a person instead of a cop for a minute, she's doing more than letting Jen off the hook. She's looking at the whole picture, from her childhood experience, to the larger crimes at hand, to Jen's motives, and her own heartbreak.