[Warning: Spoilers ahead for the Chicago P.D. season two finale, "Born Into Bad News."]

Lindsay (Sophia Bush) has been on a downward spiral ever since Nadia's death, but she did something truly unthinkable on Wednesday's season two finale of Chicago P.D. She quit.

"The reason Erin is a cop [is] because she wants to personally help people; she wants to know at the end of the day who she was able to help and what she was able to do," Bush tells The Hollywood Reporter. "Being hit on such a personal level — this really hits the core of that and shakes the ground out of from under her."

The episode's final moments, in which Lindsay told her boss and father figure Voight (Jason Beghe) that she was bad news and gave back her badge, was just as much of a shock for the actress as it was for the viewers. "The notion that she would even hand her badge over is something that I had to talk to the writers about a lot as we were developing what would happen to her and what this struggle would look like," says Bush.

So why did she do it? Bush spoke with THR about how Nadia's death "ruptured" Lindsay, why she turned to her estranged mother and what's ahead for season three.

Erin has been struggling with Nadia’s death for several episodes. Why do you think she went so far as to quit?

Sometimes it's the strongest people who fall the hardest. They're the people who you don't expect to be knocked off their game. This kind of a thing, what it brings up for Erin is where she comes from. The way that she grew up told her who she was going to be, what she was going to be able to do, and then by luck, hard work and the grace of being arrested by the right detective, she made it out. It was against all the odds and she's been a tough woman who's been able to push forward no matter what. Erin's motto is always a forward march and she always gets the job done. This just ruptured her. It sliced the parachute and then she's just falling and she doesn't know how to get out of it and how to get away from it. This notion, for her, is that maybe she's not as strong as Hank. Maybe she wasn't able to save someone the way he saved her. Maybe she should have never tried. If she hadn’t fought so hard for this girl, maybe this girl would still be alive. The notion that you caused a death or put someone in the line of fire is so brutal. That all really informed just how much this would make Erin lose her mind.

We've seen her quit P.D. before, just earlier this season when she took another job. She ended up coming back pretty quickly. How will this be different from that?

The initial storyline for Erin going to the task force was supposed to last for eight episodes. The way it changed the chemistry of the show — everyone hated it. They pulled the plug on it really quickly, but there's something about this that's different. … This left her, emotionally, in a way that I think will not be so easy to have a quick turnaround from. Leaving the task force is simply about, "I don’t like the politics. I don’t like being surrounded by a bunch of misogynistic assholes." This is getting to the very core of who she is as a person and it slays her vulnerability open in a way that I don’t think she knows how to cope with. I'm hoping at least that that's not something that when we come back in season three, everything's OK. I think she's going to have a lot to deal with when the next season of the show picks up and we'll have to figure out how to tap back into her strengths.

You talked about how much Lindsay loves helping people and how much she loves being a cop. How much will she change as a person when she doesn’t have this job grounding her?

That kind of breakdown of your psyche is the thing that I'm looking forward to delving further into as the season picks up. We're a little bit unsure as to what the time jump will be. Last season it had to be about six weeks. Now the writers are talking about, will it be six days? Or six weeks again? What will it look like? What are the ramifications? If it is more than a month, how far has Lindsay gone? Where will we find her? If it's six days, is she just going to be on a little bit of a bender? At this point, nothing is set in stone. It's really a conversation but we're certainly picking apart all of the options and trying to figure out what will be best. There's a part of me that would like for us to pick up the day after she quits and see what happens. Hopefully it will take a couple of episodes for her to want to go back. As much as I agreed that she would make a very immediate, cutthroat decision about the wrong working environment at the task force, I would like for this to not wrap itself up so quickly.

The Voight-Lindsay relationship has changed a lot in the wake of Nadia's death. Why do you think that is?

I think with people who are close to you, when you don’t feel like yourself, you don’t know how to communicate that and you don’t know how to admit it. For Lindsay, she's not good at revealing her feelings; she's not good at exposing her vulnerability. She's trying to continue on that forward march, and just can't do it. She's consumed by all of this guilt, all of this rage and feeling like she should have just done something differently. I don’t think she knows how to explain what she's going through to Voight or even to Halstead, because if she tells someone how she feels, she's going to have to know she feels. Instead of being angry and drinking and being in this "F— it all" place, she would actually have to look inward and feel that pain and feel that grief. That's a very, very hard thing for someone who's programmed to always move forward. To stop and exist within your feelings and be overcome by emotions that feel like they're going to break you is not easy for someone like her. She's very good at being courageous for other people but the only really courageous thing she can do for herself in this situation is process these feelings. But she can't do it. She doesn't know how.

Lindsay and her mom have had such an estranged, up-and-down relationship. Why did she turn to Bunny out of all the people in her life during this time?

That was something that I had to talk to the writers a lot about to really be able to feel like I could play that and believe it for myself. The moment it began to make sense is when Halstead says, "Hey, we're going to go to Molly's. We're going to pour one out for Nadia with everybody." She's looking at him and she suddenly feels like she's on the outside of her whole universe because as sad as they all are, as much as they're grieving a friend, Nadia was Lindsay's protege. Nadia was Lindsay's roommate. She was, for all intents and purposes, like her little sister. Everyone around her seems to be able to process this and she almost feels like she's on the outside of a fish bowl and everything inside is untouchable and foreign. This notion that she's just supposed to go to this bar where she usually has a lot of fun with her friends and mourn someone — it feels impossible. She doesn't want go to home because all of Nadia's things are still in her apartment; she still has her room there. But she doesn't want to go out with these guys and so where is she going to go?

In this place where she's consumed with this self-loathing, there's nobody that makes Lindsay feel worse about herself than her mom. Going to see Bunny, she doesn't have to act like she's OK and she doesn't have to be nice and she doesn't have to do anything. She can sit there and she can be around someone that makes her angry and a person that knows that they make her angry. It almost allows her to let some of that anger out because she can snap at her mom and her mom is not going to care. It's a little bit of torture. It's emotional cutting. It doesn’t have to make sense because nothing makes sense to her right now. When I envision Bunny being a physical personification of a razor blade for Lindsay and her just going to town on her heart, that’s what makes it all make sense to me.

Do you think Lindsay will ever be able to get over Nadia’s death completely? Will she ever be able to have closure about this?

I don’t think you ever get over a loss like that. But the way she's blaming herself isn’t serving her and it's certainly not serving Nadia. Something really horrible happened to someone that she cared about, but it wasn't her fault. Hopefully as time goes by, and as she actually allows herself to feel her feelings and let that grief out and deal with it, she'll be able to understand that and hopefully it will further inspire her to save the ones that she can. You can't save every single one.

Chicago P.D. returns in the fall on Wednesdays at 10 p.m. on NBC.