My guess is that as the ethos members can't attack each other they made so to move MC in a specific place where something else will kill him, the cloack could be cursed instead of giving protection.
Even if everyone the confronted MC is dead the honest people didn't, getting information on him from them should be good enough to find his real identity.
Could even be a coincidence and they decided to kill all Ethos memeber,s notice how everyone but the summoner is wearing that cloak, again, they can't kill each other, but planning it is another thing.
If they believe MC to have somehow joined them it would be perfectly logicial to kill everyone to be on the safe side.