It's gotta be the Imperial family. At the very least a Duke or close relative that stands to gain the Kingdom from the Prince's demise. Another option is for the Nobles who wish to limit the kings authority, but dealing with demons and killing their first born son, the Crowned Prince, is like taking a Flammenwäfer to light a candle. It makes no sense logically speaking. The only one to gain from it would someone next in line after the Prince as of that moment.