Probably because Pine will just disappear from the world, and also because the money was on Tradepass' account, which is still Roper's company (Birch being a strawman as Roper explained). So it is still Roper who owes the money. Heck, he would even ridicule himself by saying that Birch took the money. He is supposed to be a badass arms trafficker who knows what he does. This is serious business. Plus he insulted the guy. Even if the buyer would let Roper live, he would be out of business and no one would ever work with him again.