The death skill also appears to targeted to a specific enemy, rather than impacting a group. That's useful when you don't want to act like a stereotypical dragon and level everything in your path.
I realized that, if he does kill the hero here, it's only going to raise the MC's notoriety. The hero has someone high in not-catholics' hierarchy backing him. Once the hero is dead, it'll be much easier to mourn him for propaganda than to spread the news of a friendly evil dragon taking out a corrupt church official. Only a select few will continue to know the truth about him.
The best case for the MC's image is if the hero barely survives, goes on to do other evil things to the point it can't be covered up, and they end up relying on his help in some way. That's actually a much worse outcome for him than getting tf out and waiting for his next evolution to cover up his identity. He's at 57/75, so the equivalent of a few more ants' nests at most should get him there. The next evolution could come quicker.
I'm not sure how much longer the hero will be in play in the story, but he's so over the top evil that I'm losing interest in him beyond this arc.