One follows the other. Those people would haven't suffered (as much) if there was drug trade. They suffered because there was drug trafficking. Killing these guys does very little to nothing to halt drug trafficking, thus the endeavour is fruitless and a waste of resources, while also courting the very real risk of eventually executing an innocent.
The same follows in the next point, your contention is with the illegal recreational drug industry, which derives its power from the illegality. Remove that element and you curtail power. The indication of chemical classification is a quick way to show how absolutely arbitrary the standards for "what an illegal drug is" are, thus adding to the reason to get rid of them.
A lot of people are harmed for plenty of reasons, it is up to us to pick the method that most decreases harm. Given that the death approach involves executing people via an imperfect justice system (which carries its own risks, as already mentioned), and that you'd be hard pressed to find studies that back repressive approaches as the most effective ones, alternative approaches should be pursued.