From the moment Prince Vann summoned his gold dragon, Leo felt like he will be able to tear down those limits of his soul orb once he consumes the gold dragon's core, which means, eating Prince Vann.
He also felt the same towards the black wolf but he knew that he will die the moment he tries to kill the boy. So, Leo targeted Vann, instead.
However, there's also a huge issue with this prince of Hildegard.
One is that he's a mid-rank-6. So, he won't go down without a fight.
The second is that his Arcana spirit is a giant dragon. So, it's easy to cause a commotion.
And finally, he's the crown prince of his kingdom. Meaning, it's hard for Leo to battle Vann alone without any audience.
For him to reach his goal without getting caught, Leo needs to solve the above three issues.