A very early one as Nolan and Talia misread a domestic abuse situation and thus are unable to prevent the wife from killing her husband via stabbing. Nolan blames himself and Talia is unable to be there for him because she feels her own guilt at failing to misread the signs.
1x03 "The Good, The Bad, and The Ugly"
We discover Officer Bradford's wife is a former undercover narcotics detective who got addicted to heroin on the job before being quietly fired.
1x04 "The Switch"
Nolan breaks up with Chen in order to save her career, only to find she's devastated by this due to having far less life experience and the relationship being a lot more meaningful to her than Nolan.
1x08 "Time of Death"
Nolan has to deal with the first instance of him killing a man on the job. The action leaves him questioning everything he's done up until this point and whether or not the shooting was justified. Everyone also struggles with treating it as a criminal proceeding.
1x07 "The Ride Along"
Rupert Payne tries to pay for a mentally ill young man's treatment, only to severely misread the brother's feelings regarding the matter, and getting himself robbed as well as shot in the process. Nolan has to reassure him that he wasn't doing it from a place of prejudice.
1x16 "Greenlight"
Nolan cradling Captain Anderson's body after white supremacists killed her during a struggle, unable to do anything to protect her. And of course, the guilt he felt afterwards, as he starts to blame himself for her death since she was put into the situation by going on patrol with him.
A young boy finds a gun discarded by a fleeing criminal and proceeds to accidentally kill his mother while holding it. West is devastated and (justifiably) blames himself.
2x11 "Day of Death"
Chen sobbing uncontrollably in Bradford's arms when she is rescued from her kidnapping.
Season Three
3x05 "Lockdown"
West telling Grey that his No-Holds-Barred Beatdown was worth it if it means getting his racist T.O. Stanton out of power. And it isn't even enough to get the guy fired; all they can do is put him on administrative leave.
3x11 "New Blood"
Chen is overjoyed to hear that her parents are coming to celebrate her promotion to P2, thinking that they have finally come around to her being a cop. Her mom arrives by herself, explaining that her husband was at his book club. While Lucy is disappointed, she tries to make the most of it with her mother. Mrs. Chen then reveals that she didn't come to celebrate her promotion, but to tell her that she and her father still don't support Lucy being a cop, even though Lucy has accomplished so much and loves her job. This causes the conversation to get heated before Lucy, who is on the verge of tears, orders her mother to get out of her apartment. And if that wasn't bad enough, Lucy had spent hours preparing their favorite meal.
Season Four
4x01: "Life and Death"
Lopez and West are kidnapped by criminals hired by La Fiera. In the struggle, West is shot dead by one of the assilants,
4x09 "Breakdown"
While Tim and Lucy help his sister renovate the old Bradford house, they discover a pistol hidden in between the walls. Tim initially suspects his dying father killed the neighbor because the latter found out the former was having an affair with his wife. But the end of the episode reveals the neighbor's wife did it because he was a wifebeater, and Tim's father helped cover it up because he felt that the neighbor deserved it. Tim visits his father one final time and calls him out for being an abusive hypocrite who completely ignored the fact that he was hurting his family the same exact way the neighbor was hurting his wife.
4x11 ''Endgame"
A homocide victim that Lopez discovers is a friend of Tamara's. Considering that the girl had already lost her parents, Lucy struggles with trying to deliver the news to her, as Tamara is the only lead they have to figure out what happened to the victim. Tamara is at first is in complete denial when Lucy tells her, but becomes absolutely devastated when Lucy tells her that they saw a picture of her in the victim's phone.
Even worse, the person that killed Beca was their friend Declan, who discovered that she had stolen a watch that she planned to sell in order to get money to back to school. Declan, who couldn't bare the thought of her leaving him by himself in the streets like Tamara did, tried to sneak the watch from her, which led to an altercation that resulted in him accidentally killing her when he pushed her to the ground, striking her head. He is left in tears when he tearfully confesses this to Tamara, who is utterly horrified.
