I need a motive. Basically, there's this hidden assassin's guild, and the assassins kill anyone who finds the guild. My main characters stumble upon the guild, but instead of killing them, the assassins knock them out. The assassins plant bodies that just look like the characters, to make other people think my characters died. The assassins take the characters to their jail. Why don't the assassins kill the main characters, and why do they plant bodies in their place?
Thank you in advance for your time, sorry, I know this is a very specific question.

The assassins have a secret agenda, a reason to keep the main characters alive. Do the MCs know something? have special skills or knowledge that the assassins need? The assassins don't want "other people" in their society (non-assassins??? peasants?) to know they have ulterior motives.