it's called sound judgement.. rather than dying because larvae eat them from inside out,, he ended their suffering,, painlessly.. i prefer same thing for me.. it's disgusting to know something is inside me growing and waiting to devour me.. yuck.. so Mamiya cant really be called merciless. he's the strategist.. he's got the brains.. he made the best decision... you may see it as cruel but it is necessary.,,,
He will make the rest of them sit up lol and maybe somehow convince Touya to get his jealously under control while preventing Ruou and Shana from raising hell. Like one of the previous commenters said there needs to be someone in the group that's not in love with Kouichi lol. And he seems like the type of person that would go with his head rather than his heart.

Daamn Mamiya is hot.... I can't wait for him to join them .