why didn't the teenager be cared for or left alone
Probably because almost everyone else was already dead and because the living were starving and facing death.
She probably was cared for. However as most of the adults and children died off, either she died despite the care or she was perhaps orphaned and other starving and dying adults were unable to provide sufficient care. They had no food and very little knowledge or resources to deal with disease.
According to PBS
Only 60 of 500 colonists survived the period, now known as “the starving time.” Historians have never determined exactly why so many perished, although disease, famine (spurred by the worst drought in 800 years, as climate records indicate), and Indian attacks took their toll.
How did the teenager get "hunted" down.
She likely was not hunted down but died from disease, starvation or as a casualty of warfare between invaders and original inhabitants.
How did cannibals in Jamestown obtain bodies for food?
They were almost certainly not cannibals by choice or preference. Although they had no food, they had no shortage of dead bodies. They would have been surrounded by dead bodies, either in the open or in fresh graves nearby. As the dead overwhelmed the living, the later graves may have been only shallow.