Sometimes it works the other way. The main character is the main character because they survived the killer.

Other times it's because the killer is too attached to the main character and gives a reason like they want the MC to suffer more or they need the MC to understand why they've been targeted. In some romantic suspense, the killer doesn't plan to kill the MC, just everyone around (usually) her so she has no one else, and they can have her to themselves. Then when she doesn't cooperate, then she has to die.

Sometimes it gets kind of convoluted. And sometimes, especially in true crime cases, it can be scary how the killer formed this fantasy in their head that revolves around them getting everything they want.