I think it's mad. This in an inhuman punishment.
I am, in fact, against the death penalty in general. But for dealing drugs? Yes, heroine is a massively harmful substance. Cocaine can also be pretty harmful, when used regularly. But if there's a demand, there'll be a supply. People get into dealing drugs for some extra money, for a job, whatever, and for some it escalates - very few really realise the harm caused by peddling this stuff.
As for the idea that the production of drugs always involves violence exploitation - fair enough, it usually does, to the extent that it is unwise to buy drugs you don't know are ethically sourced because the chances are you are encouraging that side of the industry. But I have grown both marijuana and opium poppies myself for personal consumption, and there was no violence involved (aside from what I perpetrated on myself).
I am absolutely against the recreational use of any drugs, including alcohol, by anybody. But I am also in favour of the legalisation of all drugs, because that will drastically reduce violence and suffering.