In some cases, it could be a result of a mental breakdown. If Bella in the fanfiction has been through a great deal of stress or trauma, she might lose control and kill a human accidentally or without fully realizing the consequences. Also, there could be an evil influence, like an evil spirit possessing her, which forces her to commit the act against her will.
It could be revenge. If the human had wronged Bella in some way, like harming her family or friends in the fanfiction world, she might be driven to kill out of a sense of vengeance. Or perhaps she is forced into a situation where killing seems like the only option to protect a greater good, like saving a whole community from a dangerous human.