Quote Originally Posted by DEX
It's not an unfortunate misunderstanding. Both sides are being intentionally misled. The Sages even edited Rimuru's message.
When discrepancies come up (like when one side thought the priest was killed), no one stops to actually question the discrepancies and everyone just assumes the other team is here to fight.

The message was sabotaged, but the dialogue between them thus far were just assumptions and misunderstandings. All they have to do is stop and verify each other's understanding.