He covers up a slave trader knowing exactly what that person did - it seems to me like a quite legit moral reason to behead that asshole along with main target
Oh definitely, I completely agree with you on the external side of reasoning this out, I was just trying to judge him as Vlad would see him in-universe: you've got the obvious evil, the murderer and slave trafficker, who obviously deserves to die; then you've got...
some fuckin guy who came here essentially as a messenger to do a job assigned by his boss. Sure, he's protecting a fucking monster, and
we're fully aware of the entire situation and every character's angle in it, but for all Vlad knows this random priest might not have even initially known that- maybe he was just told, "a man who made a small mistake needs our protection & has made amends with our faith, we just need you to deliver the message" by that scheming fuck of a Bishop, or any number of similar scenarios. The priest is there on orders from above, after all, and he says as much as well. If Vlad knew how much trouble he was about to get into he probably would've understood the priest was a fuckin shitheel & killed them both, but lacking that kind of knowledge all he knew was that there was one man he knew he
needed to kill and one man slightly interfering... and said man isn't, like,
physically interfering, he just delivered a piece of paper that Vlad doesn't give a fuck about & won't stop him from physically gutting the bastard, just make things a pain afterward. So, again, from a broader perspective (ours) you're right, but Vlad's view of things at the time wouldn't paint the dude as someone worthy of death.