Bella was extremely thirsty, and extremely awkward, and extremely aware of the two wolves behind her. She knew she had no clue what she was doing, and no matter what she did she'd look incredibly stupid… but she could hardly ask either one of them how a vampire hunts. Especially since from their point of view, a vampire would be stalking entirely different prey.
She wandered around the forest for a while, listening intently and following her nose, as it were. Unfortunately, she had a better understand of how to use a compass than how to follow her nose.
Eventually she heard a strange noise, and turned sharply to her left to investigate. She raced along for a short distance, Sam and Jake following behind. She came out in a clearing, stopping short when she saw the bear.
It was about the same size as her companions, and it watched her as intently as she watched it. She tried to imagine how to go about… killing it, but she'd never killed anything before in her life.
'Emmett hunted bears,' she thought. The bear stood on its hind legs, perhaps hoping to intimidate her. Instead she found herself amused. It kinda reminded her of Emmett, and she was struck with the utterly insane notion of challenging the bear to a wrestling match. A wrestling match that would, hopefully, end with its blood in her mouth.
As soon as her mind touched the idea of the bear's blood, her mouth watered with venom, and her instincts took over.
Jake saw the change. He saw his love go from being the nervous, awkward girl walking through the woods, to the perfect killer. Her movements stilled, her confusion became confidence. She slid back into a crouch, and Jake realized that up until now he'd seen her as the same person. He'd known she was dangerous, but knowing and believing are two separate things.
Up until now, the only change he'd really registered was that now she was indestructible.
When she launched herself at the bear, he realized she'd also become the destroyer.