Like most shows that followed The Sopranos, Lost was filled with anti-heroes. While there were some traditional "good" and "bad" characters, most of them fell somewhere in the middle.
This not only made the series feel more realistic, but it also helped the cast come across as more believable. Too often, TV characters (specifically the protagonists) are portrayed as perfect, uncompromising heroes who never fall into temptation - or any other vice, for that matter. But not so with Lost. At one point or another, all the characters performed bad things - even the morally righteous Jack Shephard.
Jack: Tried Killing Locke
Jack is typically depicted as the "hero" of the show. He serves as leader of the castaways and always sacrifices himself for the good of the group, often venturing out into the jungle to defend them against some external threat.
However, even Jack has a dark side. He can prove incredibly emotional, and his worst moment comes in the season four premiere, "The Beginning of the End." After Locke kills Naomi, Jack confronts Locke, knocks him to the ground, takes his gun, and attempts to shoot him in the head. However, the gun is out of bullets - a fact that a horrified Locke shares with the irate Jack.
Locke: Murdered Naomi
Locke is one of the biggest antiheroes in the show. While he often proves a fan favorite, many of his actions could easily be deemed selfish, stupid, and sometimes even downright evil. Locke is devoted to no one but the island, and in season three, he abandons the castaways to get answers from Ben.
Locke's devotion to the island clashes with Jack's desire to get off it and these differences come to a head in the season three finale. Once Naomi makes contact with the freighter, Locke murders her in cold blood by throwing a knife into her back.
Kate: Murdered Her Own Father
Kate certainly had a troubling past. Many of her problems stemmed from Wayne Janssen, whom she believed was her stepfather but was later revealed to be her biological father. When Diane officially made her relationship with Wayne official, she and Kate were often subjected to physical and verbal abuse. He would also make sexual advances towards Kate, much to her disgust.
In the end, Kate took matters into her own hands and killed him once she discovered the truth about her paternity. It was this murder that eventually led to her status as the island's fugitive.
Sawyer: Killing Various People In Cold Blood
Despite being a fan favorite, Sawyer is not a good person. At least not initially. Like Kate, he begins the story as a murderer, having killed shrimp stand owner Frank Duckett in cold blood after believing him to be the real Sawyer (the con artist who led to his parents' murder-suicide).
Later, Sawyer killed an unarmed Tom after he had already surrendered, telling him that it was retaliation for kidnapping Walt. This action upsets Hurley, who looks at Sawyer and tells him with shock creeping into his voice, "Dude, he surrendered..."
Sayid: Attempting To Kill Young Ben
Sayid was one of the biggest anti-heroes on the show. While he generally means well, his past is marked with betrayal and death, and his on-island actions always remain questionable at best. Things take a very dark turn for Sayid in season five, as he begins plotting the death of young Ben.
After the castaways go back in time, Sayid attempts to alter the present by shooting and killing young Ben in the past. The attempt fails and he only ends up creating the monster he was hoping to dispose of. Like Faraday says, "Whatever happened, happened."
Jin: Viciously Beating A Man
Jin undergoes a wonderful character arc throughout the show. He began as one of the least likable characters, as he often controlled Sun, treated the other castaways with hostility, and openly attacked Michael on the beach. His past is certainly no better, as he served as an enforcer for Sun's rich father.
One of his tasks involved driving to a man's house and viciously beating him half to death in front of his young daughter. He technically saved the man's life (as another enforcer was about to kill him), but that doesn't really excuse anything.
Charlie: Kidnapping Sun
Charlie was one of the most likable characters on the show before undergoing a bit of a dark period throughout season two. Suffering under delusions, Charlie kidnaps Claire's baby son, Aaron, and attempts to "baptize" him in the ocean, resulting in a vicious beating from Locke.
To get back at Locke and make him look like a fool, Charlie decides to help Sawyer steal the guns from the hatch. This plan involved "kidnapping" Sun by throwing a bag over her head and dragging her through the jungle. It wasn't his best moment.
Sun: Cheated On Jin
Despite being one of the purely "good" characters, Sun still has her flaws. Perhaps the worst thing she ever did was cheat on Jin with Jae Lee. Sun had long fallen out of love with Jin, as Jin was growing increasingly distant and violent owing to his work with Sun's father.
She commenced an adulterous affair with Jae Lee - which ultimately resulted in his death. While Sun initially lied to Jin about the affair, she eventually came clean.
Michael: Killing Ana Lucia & Betraying The Group
Despite being a somewhat likable and tragic figure throughout the first season, Michael quickly spiraled and became one of the chief antagonists of season two.
Wishing to get Walt back, Michael enters into an agreement with The Others. He decides to murder Ana Lucia in cold blood (and kills Libby by accident), then frees Ben by making it look like the Other had escaped and shot him in the shoulder. This eventually leads to the capture of Jack, Kate, Sawyer, and Hurley.
Ben: Aid In A Genocide
Ben is the secondary antagonist of the entire show (second to The Man In Black), and he commits numerous horrid atrocities throughout the show. But perhaps the worst is his aiding in the DHARMA genocide.
He murders his own alcoholic father by poisoning him with gas, and when he returns to the DHARMA village, it is revealed that he had killed several other people. Ben had allied himself with The Hostiles, who had decided to purge the DHARMA initiative from the island.