It seems things are like this: Marisa&Marisa were hunting in the woods while kids were playing soccer nearby. Marisa has a flashback of her family being killed by humans, and little Marisa vows to do something to humans (I didn't really make sense of that part, so I might be wrong). Meanwhile, this kid really sucks at soccer and kicks the ball into a different direction. By chance, both Marisa and a different family where in the ball's trajectory, so everyone died.