I'm pretty sure there is a peaceful way to deal with the situation.
In my first playthrough, Lae'zel rushed to speak with them, then my character got to the conversation. I decided to let her speak and at some point when they talk about the weapon you get a persuasion check to tell Lae'zel to play along and not tell them anything. After they just leave.
That solution is still a bit bugged because Lae'zel comments you should check the bodies for a clue but nobody is dead so there is no way to progress her personal quest.
In my second playthrough I just killed them to see how hard the fight would be and that was worth it.