The witch-killing protagonist informs the villager that when he was a child a witch force-fed him vast quantities of evil candy. Because of this, he has to take these injections every day, or he will die on the spot. ... If you enjoy watching witches doing kung fu in 3D, then this movie is for you.

Why does Hansel have to inject himself?

It is revealed that because Hansel ate too much of the witch's candy, he now has to inject himself with medicine every few hours.

Does Hansel have diabetes?

Hansel (Jeremy Renner) has diabetes. This is not a throwaway character trait, not a bit of bizarre nuance in a quasi-steampunk re-imagining of a Grimm fairy tale.