Lost is a fantastic show, but it's also a violent show. It was never afraid to depict deaths, and it certainly wasn't afraid to depict its "heroes" as problematic antiheroes. Even the most traditionally heroic characters, including Jack and Sayid, faced their fair share of controversies, problems, and questionable decisions.
Like most antiheroes and villains on TV, the characters of Lost have racked up a considerable body count. Lostpedia has kept detailed records regarding each character's body count, and it's a fantastic source for those looking to get more information. It will also serve as this list's main source of information.
Ana Lucia - 4
Ana Lucia was introduced in season two and immediately gained some criticism from the fanbase. She was quite infamous for her tough attitude and cold personality - an attitude that resulted in numerous deaths.
Off-island, Ana Lucia murdered a man named Jason McCormack in retaliation for his shooting her and killing her unborn child. On island, Ana Lucia killed a female Other and Goodwin Stanhope in "The Other 48 Days", and accidentally killed Shannon at the end of "Abandoned". Ana Lucia was in turn killed by Michael - one of his two.
Claire - 4
Claire was one of the most wholesome characters on the show, which is why her evil turn in season six proved so surprising (and controversial). Claire had been infected with The Sickness off-screen, which turned her into a violent and paranoid murderer.
All told, Evil Claire murdered four people - two of Widmore's goons (as seen in "The Candidate"), Justin (in "Lighthouse"), and Aldo ("What Kate Does"). She also attempted to kill Kate in "Recon" but was unsuccessful.
Mikhail - 4
Veering away from the protagonists is Mikhail Bakunin, one of Ben's top goons. Mikhail served in the Soviet Army and may have killed people during his time in war, but that is unknown and never addressed in-show.
However, what is canon are his four deaths (five if counting himself). In "Enter 77", Mikhail kills Beatrice Klugh at the Flame. But his big spree comes in "Through the Looking Glass", which saw Mikhail killing the Looking Glass goons Greta and Bonnie and fan-favorite Charlie Pace.
Sawyer - 5?
Sawyer's body count is quite hard to judge. Directly, Sawyer is responsible for five murders. He killed Frank Duckett in cold blood, believing him to be the real Sawyer who ruined his family.
He has also killed two Others, Anthony Cooper (the real Sawyer), and Tom Friendly. However, things get a little murky. It was Sawyer who inadvertently caused the submarine explosion that killed Sayid, Sun, and Jin in "The Candidate", and he also buried Nikki and Paulo alive after thinking them dead owing to their spider-induced paralysis.
Charles Widmore - 6
Despite receiving a late introduction, Charles Widmore played an enormous role in the island's history and mythology, and his battle with Ben provided much of the series' action (having been introduced at the end of season three).
Widmore has officially garnered six kills. In "The Last Recruit", Widmore directly killed five Others. His sixth kill comes in "Jughead", when he snapped Cunningham's neck.
Jack - 6
Despite being the "hero" of the story, Jack has racked up a considerable body count. First, Jack euthanized Edward Mars after Sawyer shot him in the lung. How he did it remains a mystery, but he likely strangled him.
A good chunk of Jack's kills comes in "The Incident", as he killed three DHARMA in the Barracks and one more at the Swan site. Finally, Jack was the one to finally kill The Man in Black. However, it's also interesting to note that Jack tried killing Locke in cold blood in "The Beginning of the End", but the gun was out of bullets.
Mr. Eko - 8
Mr. Eko lived most of his life as a criminal, so most of his kills come off-island. In fact, the only people he kills on the island itself are two Others that are attempting to kidnap various of his camp.
Off-island, Eko killed three people in "The 23rd Psalm" - the unnamed man he shoots as a teenager and the two criminals he kills in a drug deal. His last three come in "The Cost of Living", as he kills two of Emeka's henchmen and finally Emeka himself.
Ben - 14+
Ben is the show's biggest human psychopath, and he has racked up one of the show's largest body counts. Off-island, Ben killed an unnamed Bedouin in "The Shape of Things to Come" and both Matthew Abaddon and John Locke.
On-island, Ben is responsible for the deaths of Roger Linus (his own father), Martin Keamy, Caesar, Jacob, and Charles Widmore. Finally, Ben knowingly set off the Kahana explosion by killing Keamy ("You just killed everybody on that boat!" - "So?"), resulting in the deaths of Michael, Jeff, Hendricks, and who knows how many others.
Sayid - 17
Sayid has a bigger body count than the show's secondary antagonist, and his is by far the highest of the main castaways. Sayid was responsible for numerous deaths off-island, including Omar, Ishmael Bakir, Peter Avellino, Elsa, and Ivan Andropov.
On-island, Sayid is responsible for the deaths of Diane, two Others, two of Widmore's henchmen, Jason, Erik, Dogen, and Lennon. Sayid also tried murdering young Ben after traveling back in time, but this proved unsuccessful as one cannot change the past.
The Man In Black - 47
Serving as the primary antagonist of Lost, The Man in Black has also racked up the highest body count by a considerable margin. The Man in Black is responsible for upwards of 47 deaths - most of whom were killed through his Smoke Monster form.
Of his major kills, The Man in Black disposed of Eko, all the characters who died in "The Candidate" sub explosion (Sayid, Sun, and Jin), Zoe, and finally, Jack. He also planned the death of his brother Jacob and used Ben to carry it out.