Killing a Character
Question: What are some valid reasons for killing off a major character? In what ways should the event contribute to the plot if I wanted to do this?
Let's begin with the Essential Plot Elements.
Sometimes, the death of a character is a Forewarning of what might happen to everyone if the Goal is not achieved. It shows the characters how important it is to achieve the goal, giving them motivation.
Sometimes death is a Cost, a hardship that the characters must endure for the sake of achieving the goal. In this case, it shows the reader how important the goal is to the characters, that they are willing to die or accept someone's death to achieve their aim.
Sometimes death can be a Requirement, if a death is an essential step on the way to achieving the Story Goal (though it usually needs to be a morally acceptable choice).
Sometimes death is the Story Goal. For instance, if you have an evil tyrant on the throne, the goal might be to kill him and so free the kingdom. Or you might have a story where the hero must kill the villain to avenge the wrongful death of another. Conversely, death can be the Consequence, as in classic tragedies where the main character dies in the end.
Sometimes, a murder is used just to show how evil the villain is, so the reader knows whose side to be on.
A death can be the inciting incident that prompts the characters to discover the truth (Goal) about the past or about the toxic or hidden relationships in the community. Murder mysteries often work this way. Another example is the comedy film, Death at a Funeral
And sometimes the death of a Guardian is necessary so that the main character can stand on his own two feet. If the story is to have a successful outcome, the main character must achieve the Goal by his own actions or decisions. If his parent or mentor solves the problem for him, the story doesn't work. Characters like Obi wan Kenobi or Dumbledore have to get killed so that main characters like Luke Skywalker and Harry Potter can step up to the plate.