It always has been on their best interest to not kill users, the toxicity of the supply has several reasons and a major one is aiming for high potency to make more money from each gram so that the business is viable even with logistic losses.

So just as there’s an incentive to make it less “pure” (by mixing with filler), there’s an incentive to make it more “pure” (reducing the prevalence of the strongest opioids), both reducing the potency. And the counter incentive is, ironically, crackdown. The more drug busts and logistics disruptions they have, the higher the incentive is to increase potency (and overdoses).