The CW's The 100 proved from the start that it was willing to kill characters. Two delinquents that died before the Dropship door opened were proof enough that it was a cruel world. As time went on, the deaths did not let up, and even if they were not all main characters as time went on, there were still many lives lost.
Enemies and allies are lost often, and it is not uncommon to find a familiar face in danger of losing their life. While every death is heartbreaking, some did more to affect the story than others.
Updated by Amanda Bruce on October 12th, 2020: As The 100 brought its post-apocalyptic story to a close, the series racked up an even higher body count. Over the course of seven seasons many deaths changed the course of fate for the characters. Some deaths shaped their outlook on events while others made them targets. This list has been updated to reflect those. Note that there are spoilers for the final season of the series.
Atom
Atom might not be a recognizable name for a lot of fans, but his death was actually incredibly significant for Clarke Griffin and Bellamy Blake. Atom's was the first life Clarke took in the series — after he begged Bellamy to kill him.
Caught in acid fog in the woods in the first season, Atom suffered incredibly. He begged for Bellamy to end his suffering, but it's Clarke who took a knife and did it. She tried to give Adam some peace, humming "All The Pretty Horses" while she did, a moment that came back around in the show's final season.
The Culling
After the 100 teenagers were sent to Earth, the Ark was still in trouble. It didn't have enough oxygen to make living there viable. There was a plan by the space station's government to simply get rid of some of the population, but that's not exactly what happened.
Instead, when the viability of the Ark was made public knowledge, the people on board showed some of the best of humanity. 320 of those who lived on the Ark chose to sacrifice themselves so that the rest of humanity might have a chance to survive. It's one of the first moments in the series that shows there's hope in spite of the darkness.
Russell
Russell Lightbourne lived many lifetimes as a Prime on Sanctum. Though he initially had noble intentions, he eventually became obsessed with power and his own guilt at killing his family during the first Red Sun. Though Clarke might have wanted him dead, she wasn't the one to take him out.
Instead, that was Sheidheda in Russell's mindspace. Eliminating Russell changed the big threat to humanity in an instant. Sheidheda became the big bad, disrupting the storyline at Sanctum and threatening the very existence of the human race by interfering in Cadogan's desire for ascendance in the final season.
Bellamy
Bellamy had one of the most interesting character arcs of the entire series. He was selfishly motivated when he debuted. His only goal was to make sure his little sister Octavia was safe and that he stayed out the Ark's prison. Eventually, Bellamy became a hero, and he wanted to save all of humanity, ending the wars he constantly found himself swept up in.
Bellamy died by Clarke's gun when his newfound faith threatened the life of Madi. That death changed the dynamic of the group of survivors at the center of the show in a small way: they began to question what they knew. Though they all still fought for one another, and they all strived to make sure the people they cared for most survived, they also all began to truly wonder about the possibility of ascension and what it might mean. Bellamy's death, though hated by fans, made his friends think about what was next instead of what was immediate.
Cadogan
Cult leader or genuine believer? It was hard to tell with Cadogan, right up until the end. He seemed to believe what he was saying, but he also seemed more interested in pretending to have the right answers and reconnecting with a memory of his daughter than he did actually ascending. None of that ended up mattering since Clarke shot him point blank in the middle of his final test.
Cadogan would have taken the final test to seal humanity's fate if Clarke hadn't straight up murdered him, justifying her actions as "justice" for Madi. If Clarke hadn't killed him, however, Raven never would have been able to step in and get humanity a do-over. Cadogan's death made sure that it wasn't just the voices of Cadogan and Clarke speaking for everyone this time.
Wells
For shock value or not, Wells Jaha's death made it clear that The 100 was willing to kill off significant characters. As the Chancellor's son, Wells was already an enemy to several delinquents as his father had floated many of their parents. Wells was a constant reminder to several of the hundred of what they hated about the Ark. Wells' death instigated hanging Murphy for his fate, even though Murphy hadn't killed Wells.
Charlotte
It was Charlotte that killed Wells. Young and scared, Charlotte saw Chancellor Jaha in her nightmares and had to face Wells when she woke up. Although Murphy was initially blamed for Wells Jaha's death, she confessed to his murder as the delinquents hung Murphy, giving Clarke enough time to cut the rope and save him.
After being hunted by Murphy, and guilty for her actions, Charlotte jumped off a cliff. Charlotte's death led to the alliance between Clarke and Bellamy when they decided to make the rules together instead of running their camp based on chaos.
Finn
Finn was not the first person to die at Clarke's hand, but he was the first to make such a massive impact. Finn had sealed his fate after murdering several innocent, unarmed Grounders in their village. Lexa's initial request to agree to an alliance was that Finn dies for his actions. Clarke's choice to mercy kill Finn saved him from going through excessive torture and pain. With Finn's death, an official partnership formed between the Grounders and Skaikru to save their people from Mount Weather.
Josephine
Josephine Lightbourne did not make her first appearance until season six. However, her death was the event that sent Russell and Gabriel on a journey to find a way to bring her back to life. Eventually, they succeeded in bringing Josephine's mind back to life through another person's body. The discovery leads to the existence, and ultimately immortality of the Primes. Season six emphasizes the horror of Josephine's immortality as she briefly takes control of Clarke Griffin's body.
Conclave
The Conclave caused the deaths of several, but given the circumstances, it still counts. There is one final Conclave to determine which Clan gets to survive Praimfaya in the bunker. In the end, Octavia is victorious, but the deaths of the other warriors were not in vain. Octavia declared that the bunker would be shared by every Clan, each submitting one-hundred .
The Conclave marked the deaths of Roan and Luna. As many believed that Luna was the last Nightblood, her death, and Ontari's before her, also lead the Grounders to think that the time of the Commanders was over, and they turned to Octavia to lead Wonkru.
