Depends on if the curse had other effects. If say it was stealing strength then you go with the strongest "disposable" target.
Or maybe he was starting to overshadow the "hero".
Heck considering the rest of the "hero"'s party the MC was likely the only calm and mature one, which could lead the "Hero" to fear losing his spot as party leader.
@sylvacoer totally agreed, he saw that he might lose his lead role in their party and wanted MC to die for it while making it look like he had no part in it.
i wonder what the village gonna say when they see a dragon in the air coming (even-though it's dead). The way the dragon is position might make it look like a lazy dragon(physically) that uses magic to move around.