I usually stick up for the merchant, and then the druids attack.
Which is a lame way to go

I mean, they could ask him to stop, or explain why they want to kill him... but "get out of my way or else" really doesnt do it for me.
Neither does "he is evil" (Mr Bush has turned that one into a cliche).
I honestly can't remember if the merchant gives you anything, but sticking up for him is probably the best decision