Til Murder Do Us Part

Somone is attempting to kill their spouse (or already has), either to collect on life insurance, because the voices in their head told them to do it, or for some other reason. Maybe they're really a serial killer who marries their victims and then offs them in succession. Or maybe Divorce Requires Death in this setting.

Most often the couple is newly wed, and the unfortunate victim usually won't suspect a thing until after the wedding, when things start to take a turn for the disturbing.

Compare The Bluebeard and Black Widow, who do this over and over again.

A common parody is to have the protagonist think his spouse is trying to do this, often with several Stab the Salad moments.

Comic Book

 * Played with in Batman Adventures #16, where the Joker starts reciprocating Harley Quinn's affections, and even proposes to her -- just after she receives a letter informing her she's inherited a fortune. The twist is that.
 * The really messed up part?

Film

 * Happens in the MSTed film The Screaming Skull with the husband trying to murder his new wife, just like he did to the old one.
 * In So I Married an Axe Murderer Mike Meyers thinks his new wife is one of these.
 * Humperdinck plans this in The Princess Bride to give him an excuse to invade the neighboring country.
 * The Alfred Hitchcock movie Suspicion focuses on a woman who is frightened that her husband killed his friend and is trying to kill her.
 * A good chunk of Mr. and Mrs. Smith involves the titular couple trying to kill one another.
 * Done to Dracula, of all people, in Son of Dracula.

Literature

 * There is a short story by Robert Sheckley (I think it is called "Trap"), where an alien sends a few animals and then his spouse through a teleporter disguised as an animal trap. He was tired of his wife, but they were married for life.

Live Action TV

 * Taken to an extreme degree in Monk, and somewhat less so in Psych, where pretty much every other mystery ends with someone trying to kill their spouse, oft times with little reason beyond just wanting to do it. The worst example was that one time in Monk where the husband found a suicide note his wife wrote years ago, and decided to kill his wife simply because he realized he could with no risk.
 * There was also that time the foreign diplomat was killed. Political Assassination? Someone trying to start an international incident? Nah, he picked up some guy's jacket by accident, and the guy killed him so that he could retrieve the evidence that he killed his wife, which was in the jacket.

Video Games

 * Variant in Final Fantasy X: Yuna marries Seymour as part of a ploy to get him Sent (the only way to kill someone off for good in that 'verse), she figures he'll be too... "distracted" to put a stop to her until it's too late. And Seymour himself is marrying Yuna as part of a ploy to get her to become attached to him, as he knows . So, in short, both of them are planning to off the other.
 * in Silent Hill 2.
 * Hitman: Blood Money features a mission called "'Til Death Do Us Part" wherein the player must kill the groom at a wedding. It is implied that the bride is the one who called the hit. There are instances where the groom's death will cause her to say "Finally."

Western Animation

 * In an episode of The Simpsons, Selma marries deranged serial killer Sideshow Bob, who plans to kill her on their wedding night, and then go after his now-nephew Bart.