The perpetrator was actually a spy planted by Regalia, specifically Chaotic Death. This man was a loyal subordinate of Chaotic Death, even the other Regalia's soldier didn't know about this spy.
Chaotic Death had planted this soldier when he was about to defect to Regalia. This man was also the one who caused the first attack at Primavera City. He helped Regalia's spy to open the Primavera City's gate, which caused the emergency situation previously.
Chaotic Death had seen the Second Prince coming. With the Second Prince coming, he knew the chance to get Primavera City had almost gone. Moreover, with the weird situation that happened, this made claiming Primavera City almost impossible.
However, Chaotic Death didn't want to leave empty-handed. He wanted to try something before ordering a retreat command, to kill the First Prince. That was why, he requested this loyal subordinate of his to take a chance to kill the First Prince.