Little Sweetheart gives us an attempt by the victim of the utterly evil 9-year-old Thelma, but first he assumes it's her brother, and after hurting him a bit, discovers it was her. At this point, he plans on getting the brother (who was also blackmailed by her) to get the girl to his house so that he can scare her a bit (he's a bank robber, not a murderer, after all), but then the police get in the way.
At the end of Above The Rim, gang leader Birdie (played by Tupac Shakur) meets Karmic Justice, not at the hands of the hero or his mentor, or even the cops, but his Butt Monkey lackey, Bugaloo (played by a young, twitchy, Marlon Wayans).
In the 2008 Get Smart film, Siegfried constantly belittles his Brute Dalip, considering him little more than a Dumb Muscle. At the end of the film, as Dalip drives Siegfried and his right-hand man off after their plan has failed (courtesy of Dalip tipping off the heroes before it began), Siegfried decides to make a few more jokes at his expense, upon which Dalip just punches him out the window and off of a bridge.
To expound upon this, he punches him so hard, that he and the WHOLE DOOR have 5 to 6 seconds of hang time. (Remarked upon by the right-hand man.)
Sky High: Ethan's ever-so-glorious revenge on Lash.
At the beginning of The Lord of the Rings: Return of the King, Wormtongue kills Saruman during the confrontation with Gandalf and the Fellowship. In the book version, this occurs near the very end during the Scouring of the Shire after getting mistreated one time too many by the evil wizard.
In Hannibal, Verger's unappreciated physician Cordell is spurred to Bite Back against his employer, dumping Verger out of his wheelchair and into a sty of man-eating swine, when Lecter points out that Cordell can tell the cops that Lecter's the one who did it.
At the end of New Jack City, crime lord Nino Brown - fresh from cutting a ludicrously sweet plea deal, gets gunned down by a old man he'd dissed and abused early in the movie as almost an afterthought.
Biff Tannen in Back to the Future has something like this happen to him with regards to George McFly. Although George isn't exactly his underling, Biff takes a great deal of pleasure in bullying him and making him act as his dogsbody. Then Biff gets it in his head to sexually assault Lorraine, the girl George is in love with, inspiring George to show Biff how much he appreciates this and all the other poor treatment he's been subject to with a surprisingly strong left hook. Thirty years later, the roles have reversed somewhat. While George isn't a bully, Biff tries to suck up by playing the lapdog.
At the end of Training Day, Manipulative BastardAlonzo has been thwarted, beaten, and humiliated in front of the neighborhood he terrorized and kept under his thumb. Desperate to stop Jake from taking the money he needs to pay off The Mafiya, (who are a bit upset with Alonzo) Alonzo offers to pay anyone who has been watching the fight to kill Jake. They refuse, and one of the local gangsters even pulls a gun on Alonzo and covers Jake's back while Jake walks away. As a result Alonzo doesn't make his payoff and gets killed Sonny Corleonestyle.
In Transformers: Dark of the Moon, Megatron, thanks to his injuries in the previous film, is now but a shadow of his former self. When Sentinel Prime turns against the Autobots, he practically makes Megatron his bitch. When Carly points this out to Megs, he decides to prove he's not by shooting Sentinel in the back, just as the latter is about to finish off Optimus. This proves to be both his and Sentinel's undoing, when Optimus kills both of them in the space of 30 seconds, while missing an arm.
This trope is more or less the driving force of Sweeney Todd: The Demon Barber of Fleet Street, and it happens no fewer than four times. Todd's motivation is getting back at Judge Turpin, who imprisoned him for life on a trumped-up charge and raped his wife. Todd returns to London specifically to kill Turpin and The Dragon, Beadle Bamford. And by the end of the movie, he is successful. Meanwhile, abusive asylum-keeper Jonas Fogg is done in by Johanna (in the stage show) or his inmates (in the film). Finally, Tobias, driven insane after witnessing Todd's brutality firsthand, does this to Todd.
In Singin' in the Rain, Lina forcing R.F. to drop Kathy's name from the credits of the new musical winds up becoming her undoing; R.F. decides he's had it with her and helps expose her "lip-synching" scheme.
In The Book of Eli, the Bible is revealed to be written in Braille. When Carnegie asks his blind slave/servant Claudia to read it, she lies saying she has forgotten how to read Braille.
An odd variation was done in Kitty Galore, the sequel to Cats & Dogs. Basically, the titular antagonist, after her plans have failed, ends up being launched into a cotton candy machine and humiliated by her pet mouse Scrumptious, having obviously seized the opportunity and courage to get away from her. The reason it's odd is because Kitty never actually did anything bad to him and in fact actually seemed to genuinely care for him as if he were her pet, although its likely she was too affectionate, as Scrumptious was shown quivering in fear as she's giving it affection.
In the House on Haunted Hill (1999) remake series, the ghosts of the insane asylum were tortured by a certain Dr. Vannacutt. In the sequel, after the idol was thrown into a sewer and washed away, the ghosts immediately take the opportunity to exact revenge on Vannacutt's ghost by tearing him apart, also saving Ariel and Dr. Hammer.
Done literally in the first Air Bud. Buddy, then known as Old Blue, ends up swatted by a newspaper by his former, abusive owner, an alcoholic party clown, and the movie strongly implies that its happened enough times for Buddy to be deeply afraid of newspapers, enough to attempt to bury them the very instant they arrive at his new owners house, and of loud sounds, as revealed during the climax while at court. Eventually, during the Disney Dog Fight climax, his former owner attempts one last time to get Buddy to return to him, and brings out his newspaper as a gift if he returns. Buddy turns towards the owner... and then pounces viciously at him, snatching the newspaper, and rips it apart before heading for Josh.
Done with tragic results in Goodfellas. After constantly getting harassed by Tommy for months - even getting shot in the foot - a young bar servant named Spider, finally stands up to him one night. He gets murdered in cold blood just a couple of minutes later, when Tommy guns him down. It's at this point that most audiences no longer liked the character and wanted to see him pay for his actions. They get their wish later.
Happens twice in Kung Fu Hustle both times in quick succession to Brother Sum. First when Sing smacks him with an improvised club for yelling at him to help The Beast. Then after The Landlord and Landlady escape with Sing he tries that on The Beast and gets his head knocked 180-degrees for it.
After Iron Man defeats all but one of the terrorists holding an Afghani village hostage, he tosses the boss man, already beaten to a pulp, in front of the wives, children and other relatives of the men the guy had killed to keep them in line, and then walks away. It doesn't end well for the terrorist.
Elysium: Kruger mutinies against Delacourt while she's chewing him out.
Scanners II: The New Order: In the end the Scanners who were abused and turned into dying drug addicts by Forrester and his men help the hero by fighting back to kill Forrester's Dragon, using their collective powers to incinerate him.
Magneto to Stryker in X2: X-Men United when he finally gets a chance to get some payback for his treatment in prison. An earlier script was going to have Magneto kill Stryker (which would be more in character) and the X-Men find Stryker's body when they escaped from the facility - but that was changed for the sake of letting Wolverine have one last conversation with Stryker. And honestly, Magneto probably took quite a lot of enjoyment in leaving Stryker chained up to wait for death.