It could be the enemy forces. In a war novel, the most straightforward reason for a character's death is often combat with the opposing side. They might have had a strategic advantage or just been more numerous during the battle.
Another character in the novel is responsible for Mason Verger's death.
Marcus's death can have a huge impact on the overall plot. It might change the course of the war in the story. For example, if Marcus was a key leader, his death could lead to a lack of direction among his troops, causing them to lose battles. It could also deeply affect the other characters. His friends or comrades might be filled with grief and revenge, which could drive their actions for the rest of the novel.
In the graphic novel, it's suggested that a combination of factors and individuals might have played a role, but no single clear culprit is pointed out.
In some versions, it could be a former ally who has been corrupted. This character, once on Ahsoka's side, has now turned against her due to some external influence or personal greed. Their knowledge of Ahsoka's strengths and weaknesses would make them a formidable opponent, and they could be the one to cause her death.
Well, it might be a combination of factors. Maybe it was the negligence of someone who was supposed to protect her, or a series of unfortunate events that led to her demise. It's hard to pinpoint just one person as being solely responsible.
The comic doesn't explicitly point to a particular individual as the one who killed Kurt Cobain. It keeps you guessing and leaves room for your own interpretation.
Marcus and Dom could be two friends who had a very eventful relationship during their lives. Maybe they were soldiers together, or they grew up in the same neighborhood and had many adventures.
Christine herself might be responsible in an indirect way. If she rejects the Phantom completely and coldly, it could shatter his already fragile mental state. He might then take his own life because he feels there is no purpose left for him without her love. So, while she doesn't directly kill him, her actions lead to his death.
The person behind the death of the 5 children in the FNAF story was a man in a purple suit. He lured the children into a back room at Freddy Fazbear's Pizza and committed the heinous act. His identity remains somewhat of a mystery, but his actions set off the chain of events that led to the animatronics being haunted by the children's spirits.
Another possibility is a former crewmate turned traitor. This crewmate could have been jealous of Edward's leadership and the share of the loot he got. So, the traitor could plot with other pirates or even the enemy. They could stage a mutiny at a crucial moment, like when they are about to face a big battle or are in the middle of a difficult sea journey. During the mutiny, the former crewmate could be the one to deliver the fatal blow to Edward.