Yeah, Hiiro is being a gigantic asshole here. You don't go to another person's land, learn about what has been plaguing their people for years in 5 minutes, and then tell them how they should run things. Try doing that in Japan, or hell virtually any country, and see how far that attitude gets you. The problem is, there are a lot of unknowns. The greatest warriors of this tribe tried killing this... thing... and it ... killed(?) many of their own including possibly the strongest amongst them at the time. What happens if when you kill it it just goes incorporeal and possesses you. "THAT DURNT MATTER, KEEL IT N-E-WAYS." Thank you for you input, Hiiro, you didn't even know the fucking thing existed 5 minutes ago much less what it's capable of. And telling the boy(?) to kill his own possessed father is... easier sad than done. Let's not forget that this thing killed an entire party of warriors, including the chief at the time, before it even took possession of that new body. So... There's little to no evidence that they can even take on the possessed chief and win. "B-B-But the lolibaba said teamw-" Stop. Just stop. Yes, they have excellent teamwork. She also said that BEFORE she knew that they were facing the possessed former chief and a creature that fucking slaughtered a whole hunting party of their top warriors. Notice how she wasn't fucking saying much after she found out, right?
Any before anyone tells me somehow Hiiro isn't being a gigantic shithead and busting out the mental gymnastics equipment: we've seen before Hiiro has a, shall we say, a delicate history with his dead parents. What saved him when fighting the spider was a visage of his (now dead) dying mother.
...What if he had to fight a possessed isekai'd version of his mother? (Yeah, I know that won't happen. Don't care, it's merely an example.) Do you think Hiiro would somehow be okay with that? FUCK NO! Fuck. No. And you know it. So telling someone else to do it when he wouldn't be able to... That's just sprinkles on this shithead sundae.
There are just sometimes where his character shoots off and does fucking puzzling shit that doesn't match up well with his established character. From what I know of him, what he should be doing is using an excuse to go and fight and kill this thing himself. Not this dumbshit of arguing with the tribe, insulting them and telling them how wrong they are. Because Hiiro, who usually isn't an idiot, would realize that's an incredibly fucking dumb, self-righteous and belligerent thing to do.