Killing the merchant in Professional does not remove him entirely.....
Decided to road test the theory in the beginning of the Castle level, right before I blew open the gate with the cannon. I went to the very start where the drawbridge is, saved, shot him, then went back and fired the cannon. He was right there waiting for me around the corner as usual. I reset, which took me back to the door right after you leave the first merchant at the beginning of the level.
THAT merchant stayed dead. But the other merchant right after the gate was blown (And I'm assuming the other ones) were alive and kicking.
I love to love my Lisa.