@Arkos Yes, he's a murder hobo, but he's not exactly dumb considering that he actually came up with a sound plan most of the time. It's just that he tend to miss a little detail in his plan, which is normal for a human.
And sure, he can sneak past everyone and enter the last room without being noticed. Remember that he's laying in ambush when Saito entered the room, which means he was inside the room some time before Saito's team. Have you ever considered the possibility that he was already inside with his demon bros looking for the artifact before Saito's team entered?
With that in consideration, I found that he actually knew where to look.
Also, it's been shown that the Minister has teleportation spell. Would he still have a chance to take the artifact from the Minister team should they find it first? Given the situation, that the king sent the minister to keep the artifact a secret, they would just teleport out of the labyrinth right after finding the treasure they sought. Even if they don't, they would be on full alert to keep the artifact safe. Not to mention the possibility of the artifact gets broken during the ensuing fight.
So the only chance to prevent them from doing so is to eliminate them before they even found anything, when they would be slightly lax. He might not know exactly who they were, or what they're capable of(he did know that the team was the strongest team participating in the exploration), but he at least did the necessary thing required to sneak kill them (bypassing the minister's detection magic and actually cutting the demonlord on where it would count)as well as making sure that they'll stay dead (asking the fairy to burn their corpse). Not to mention that eliminating them early would at least delay other parties. Remember that the Minister team was steamrolling the monsters inside, which pretty much opening the path for other adventurers to go deeper as the more dangerous monsters have been defeated already.
Can you suggest a better plan then? One that is not relying on the adventurer's benevolence? Or one that requires you to destroy the passage, cause it's most likely impossible to do?
If anything, he missed the little fact that he didn't know how the artifact actually looks like. Which is probably why he hasn't found the artifact yet, and why he had his two demon bros out of combat most of the time-probably he told them to look for the artifact as they are more adept in magic than him.
Yes, he's a murder hobo, but he's far from dumb. And if you consider that forgetting one or two details is dumb, then it's the same as if you're saying that most human in the world are dumb.
What I would consider dumb murder hobo (if according to your last few sentences) is if he insisted on continuing the attempt to kill Saito's party after the dwarf mage casted the blast magic on him while realizing that the fairy couldn't aid him.
And yet he chose to retreat and bring more allies instead, even sneak attacking the dwarf when he's not focused on battle.
What I would consider dumb murder hobo is if he actually attempted to kill
everyone who entered the new passage.
And yet he didn't. So far he only tried to kill two groups; one that has the highest chance to find what he wanted before he does, and one who accidentally stumbled on the place where the thing he wanted is most likely hidden in.
tldr; he IS a murder hobo, but he's not dumb or stupid, and certainly not a super genius who can plan ahead either.