There is a pretty easy way to kill the shopkeeper if you are so inclined to do that, even if its not a weapon shop. It will cost you one rope and one bomb.
What you do is put a rope just outside the door (one square away from the door). Then you go into the shop and pick up an item (don't buy it) and carefully walk toward the exit, but be absolutely sure you go up the rope and don't go past the rope even slightly. If you do you are dead. If you go up the rope the shopkeeper will follow you and end up right below you. So then you point yourself back toward the shop and then go back down and carefully place the item back on the floor - IN THE SHOP. If you set it outside of the shop you are dead. If you throw it you are dead.
Now the shopkeepers are dumb. He will just stand there in that spot at the bottom of the rope. So you walk out of the store, turn around and carefully set a bomb on the ground in front of him (but not on him and definitely don't throw it). It doesn't have to be that close so don't push it. Step back and BOOM. Dead shopkeeper. Grab everything and look for the shotgun (which probably went flying and often ends up on the level below).
This will take a few tries for sure, but you'll soon be able to kill the shopkeeper easily. If you're going for score, such as in the Daily Challenge, you pretty much want to kill every shopkeeper if possible. They drop gold plus you get items for free. Of course once you anger the shopkeepers they will stay angry and one will be guarding the exit from then on.
