In Love With the Gangster's Girl

Who is that? She's perfect. She's a vision. She's like no one you've ever seen. You've got to get to know her, to learn everything about her.

Wait. She's got a boyfriend? Oh Crap. And her boyfriend is that guy? It just got worse.

Many a tale has been told about a poor schmuck who fell in love with a beautiful woman, only to find out she likes someone very dangerous. Usually, the dumb cluck will persist in trying to win her heart and "take her away from all this". Depending on where the story lies on the Sliding Scale of Idealism Versus Cynicism, our hero may get the girl in the end and get away clean, or end up very, very dead. Or he may do the sensible thing and get out while the getting's good. But then there wouldn't be much of a story, would there?

Compare Mafia Princess.

Examples of In Love With the Gangster's Girl include:

Anime and Manga

A oneshot character in the Black Jack manga does this by accident, he's fallen in love with Pinoko's voice, and a coincidence makes him think the gangster's girlfriend is the one with angelic vocal cords. She isn't, and the would-be suitor makes a quick getaway.
A story in Petshop of Horrors: Tokyo has a small-time yakuza member fall in love with his boss's girlfriend, who he knew in high school. When the boss tries to frame the member for a crime, the girlfriend agrees to run away with the guy. It ends not so great. The guy falls into the ocean and seemingly drowns so that the girl can escape. The girl is later seen telling a police officer the story, implying that she made it. It later turns out that the yakuza member was turned into a dog and taken in by one of D's purebreds.
Tarot Cafe has a Romeo and Juliet love story between the daughter of a mob boss and the son of a rival boss. Both know full well what will happen, but choose to elope. The daughter's magic pet cat is also in love with her, but no one actually knows about it. The cat later dies so the daughter and her boyfriend can escape.

Film

Bertram Potts for Sugarpuss in Ball of Fire.
Mad Dog And Glory
Bound is a Gender Flip version, or half of one.
An in-universe one in Singin in The Rain; in the Film Within a Film The Dancing Cavaliers, Gene Kelly's character has a Dream Sequence where he imagines himself falling for Cyd Charisse, but ultimately losing her to her mobster boyfriend.
Played with in Pulp Fiction; Vincent senses the rising tension between him and Mia and explicitly warns himself to avoid this trope.
Stanley Ipkiss falls in love with Tina, the girlfriend of Dorian the gangster in The Mask. It results in a big showdown between the two of them, once Ipkiss gets the mask back. And it's Tina who decides she likes Stanley enough to leave Dorian, rather than Stanley trying to take Tina away.
The Cook the Thief His Wife And Her Lover is a good example of the "doesn't turn out well for the guy" version.
Little Shop of Horrors: Seymour is in love with Audrey, who is the abused girlfriend of a sadistic dentist.
In Dick Tracy, 88 Keys feels this way about Breathless Mahoney, whom Big Boy Caprice appropriated from one of his rivals after sending said rival to his death.
The Girl Can't Help It ends up being this. Fortunately, the gangster realizes he was just pushing his girl to help him get back his Glory Days. Once he finds his own way, he lets her go.
The movie The Marrying Man starts with the protagonist sleeping with a woman who turns out to be the girlfriend of the notorious gangster "Bugsy" Siegel. Uncharacteristically the gangster is a good sport about this and rather than having the couple killed, he has them marry each other.
The Bollywood film Main Tera Hero has the protagonist falling in love with the reluctant girlfriend of a corrupt policeman; once he frees her of that relationship, he finds himself being the romantic interest of the daughter of a mafia Don, and what princess wants Daddy gives. He manages to free himself of that by matchmaking the don's daughter and the policeman

Literature

Leads to the final fate of the main character in The Brief Wondrous Life of Oscar Wao.
The Stephen King story "The Ledge" deals with a man who falls in love with a gangster's wife and is forced to walk a ledge all the way around an apartment building. It's very high up and the ledge is six inches wide. If he wins, he gets the girl, n.q.a. If he loses, he falls to his death. This guy being a criminal, of course, there's a rather nasty loophole in the deal.

Live-Action TV

Furio with Carmela in The Sopranos. Subverted in that Furio is a gangster too, but Tony is the boss.
In the second season of Boardwalk Empire, Owen, an IRA enforcer who works for Nucky starts pursuing and eventually sleeps with Nucky's mistress Margaret. Since the show has some of the same creators as The Sopranos, comparisons between Owen and Furio happened almost immediately.
In the Star Trek: The Next Generation episode "The Royale", Riker, Data and Worf are stuck in a virtual reality simulation of a poorly written book where the bellhop at the eponymous hotel/casino is in love with a gangster's moll. The bellhop has a gun with which to "deal" with the gangster, but the gangster kills him instead.

You should have listened to me, kid. No woman's worth dying for. Killing for, not dying for.

A modern retelling of The Canterbury Tales on the BBC featured this trope in an adapation of "The Sea Captain's Tale". In the original, the cuckolded husband is a ruthless banker/usurer with a manipulative, extravagant wife. In the adaptation, the usurer is The Don of a South-Asian community and is a money lender and "importer/exporter". In both versions, the seducer borrows money from the husband on her behalf. In the modern version, the guy is more innocent, and the husband is a Crazy Jealous Guy, so things don't go well.
Castle had an episode involving a the 50-year-old diary of a gritty private eye, a scuzzy crime boss, a gorgeous gun moll, and a diamond-encrusted butterfly. That one actually turned out very nicely.

Music

In "El Paso" by Marty Robbins, both the male love interests end up dead.

One night a wild young cowboy came in. Wild as the West Texas wind.
Dashing and daring, a drink he was sharing with wicked Feleena, the girl that I love.

The song "Solomon Jones" by Aceyalone & RJD2 ends in a shootout between "Big Bad Solomon Jones" and a mysterious man who had wandered into his bar, all over "the lady known as Simone".

Then all of a sudden the music changed, and everyone just held their pulse
But it felt like your life had been robbed from you, and everything that you held close
That someone had stolen the woman you loved, and that her love was a devil's lie
That your heart was gone, and the best thing that you could do was crawl away and die
It's the painful cry of a man's despair, deep down in his bones
"I guess misery enjoys company", said big bad Solomon Jones

"Copacabana" by Barry Manilow.

Newspaper Comics

Luann. Luann's brother Brad fell in love with his next door neighbor Toni Daytona. Unfortunately, she turned out to have an emotionally abusive and dishonest boyfriend named Dirk, who was extremely muscular and could have broken Brad in half without trying.

Western Animation

The Donbot's wife on Futurama cheats on him with Bender. They're...less than subtle about their relationship.
Henchmen 21 has a crush on Dr. Mrs. The Monarch on The Venture Brothers. She's fairly dangerous herself, but her husband (guess who) is fairly cavalier about killing his henchmen if they so much as take him by surprise. He eventually quits his job when he's no longer able to handle working with her and the other, mounting psychological issues the job's causing him.