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Detective Bob is working on a murder case. He brings in one suspect for interrogation, a rather obnoxious fellow named Charlie. During the interrogation, Charlie begins making comments about beating up geeks, jamming them in lockers, and various other stereotypical bully things.

For Bob, this starts to hit home for him, because when he was in high school, he was bullied by folks like Charlie, and they always got away with it. Bob begins to let his emotions about Charlie begin to affect his judgment, and suddenly starts being much more abusive (either verbally or physically) toward ol' Charlie.

Of course, this is just one type of scenario. Basically, this trope occurs when Bob encounters events similar to something he's gone through before, and his emotions about their original encounter(s) begin to cloud and/or affect his judgment.

Compare It's Personal. Some particularly devious villains may invoke this as part of a Hannibal Lecture.

Not the be confused with the one-panel Newspaper Comic.

Anime and Manga

 * In one episode of Ghost in The Shell Stand Alone Complex, Section 9 tracks down a group of organleggers. Major Kusanagi, herself a full-body cyborg since childhood, gets much more emotionally involved than usual because one patient who was affected by the actions of the thieves was a little 6 year old girl. She even pretends to be a member of The Mafiya and threatens to kill one of the criminals in order to Scare'Em Straight.

Comic Books

 * Due to the fact The Punisher's origin revolves around the death of his family (especially his two young children) if he sees a child being harmed or at risk he tends to flashback to the day of his family's death. Then things go horribly, horribly wrong for the abusive party.

Literature

 * Commander Sam Vimes of the Watch in Discworld often relates a crime to his past of living in the ghetto. Take my advice, don't pick on the poor in Ankh-Morpork if Vimes could find out about it... not healthy for you.
 * In Feet of Clay, Captain Carrot confronts one of the plotters behind the plot to poison Lord Vetinari, which has resulted in the accidental murder of two people who lived in Sam Vimes's old neighborhood. When the plotter in question asks if the people killed were "anyone important", Carrot informs him that he should be lucky Commander Vimes wasn't around to hear him say that.
 * In Animorphs, Tobias has one of these when he spots a couple of kids bullying another boy. He unleashes what he calls a "talon haircut" on them.

Live Action TV

 * In an episode of Malcolm in The Middle, Lois is on jury duty. After viciously calling for the conviction of a teenage thief, she realizes that she's projecting her feelings about Francis on the defendant and resigns, but not until she's convinced everyone else and wasted an insane amount of time.
 * Used twice by Kutner in House. One episode has him act very sympathetically toward a girl who claims to be orphaned (like him), but when he discovers she lied, the niceness stops. Averted in a later episode, in which Kutner is mean to a teenage Jerk Jock. Taub assumes that this trope is in effect, but it later turns out that Kutner was acting on guilt caused by his own high school jerkosity.
 * Also used by Thirteen in House as she forces parents to tell a young child that he isn't biologically male but could be either sex. Related because of her own unusual sexuality (she's bisexual).
 * Sara Sidle in CSI tended to lose her objectivity if there was any indication at all in a case that a girl was being sexually abused or exploited. While it's never made explicit, it's strongly implied that she was abused herself.
 * Catherine Willows has a similar reaction in any case involving children.
 * Also played on CSI: NY with Lindsay. She had a meltdown when dealing with a case that reminded her of the attack that killed her friends, though she didn't blow up, just ran off and refused to deal until Stella caught up with her.
 * Any case in Burn Notice that involves kids with an abusive father tends to get Michael's full attention.
 * In NCIS Agent McGee is interrogating a suspect who brags about beating up nerds, he plays along for a minute or two before revealing he used to be bullied and as a Federal Agent he was now the one in power and could make the bully's life a living hell.

Film

 * In Murder By Numbers, the lead loses all objectivity dealing with one of the suspects, Richie,