Quite true but to me is best to just do the needful which is to teach that individual who wants to learn. It can only work smoothly if we can read the thought of the next person to know what he's thinking or planning to do next but that's not possible. There's nothing bad in teaching the drug dealer guy, not that if he uses it to do something illegal is your fault, all you did is showing him the best way to make money without being chased around by the cops. He might even change his mind depending on how you use your words, can get into his skin to convince him about the drugs thing
Information, like any instrument, can be utilized for good or evil.
He can say yes he'll change or not to use it for evil act but those are just words, after saying that he might go and do something different from what he said, so is not that easy to say something and abide by it. And at the same time he can be on the drug business and also on cryptocurrence business too, so we can point out his intentions.