This occurred to me, but Hermione really shot herself in the foot by publicly demonstrating the ability to successfully cast a charm above her year level on the first try, from nothing more than having glanced at the instructions once.

Plus, just from naming conventions I would expect that spell to have non-lethal uses- it's not called the Blood-Cooling Curse, or even Hex, but Charm. Maybe it's a treatment for heatstroke, or the counter to the Blood-Boiling Curse. In any situation other than under Hogwarts' wards, a Charm that takes six hours to kill an unconscious first-year isn't that big a deal, compared to, say, Levitating them off a cliff and dropping them.