@Aichan : pride, arrogance, greed, etc.
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.