It is not a reasonable argument for an individual to make to try to justify their illegal get-rich scheme.
Whether true or not, the intention behind drug laws is physical harm and addiction. Additional harms like risk of violence or overdoses due to impurities are secondary at best.
Unless there's judicial activism going on, making that admission is pretty much just handing the keys to freedom over. Why even bother with a trial? Should have done a plea bargain.
If the law is hurting people, and breaking the law helps people, it's completely reasonable to use that argument to morally justify your illegal behavior. I'd be interested to hear your argument why it is not.
Edit: Though to be clear that wouldn't work to justify all your illegal behavior, if that included like hypothetically trying to have someone killed.
I'd agree that lawbreaking is a reasonable moral choice, but not in this case. Moral lawbreaking, in my view, is done when there is a clear benefit or reason to the lawbreaking. And by clear I mean clear to a reasonable and moral person, and not just a speculative personal opinion.
Given that frame of reference, I would argue that (1) the notion that online drug dealing would lead to harm reduction for a drug addict is speculative rather than clear; (2) the harm caused by improved access to hard drugs by more potential addicts would be tremendous and therefore likely exceed any potential harm reduction; (3) given Ulbrict's apparent willingness to hire hitmen, I find it impossible to believe he was in any way motivated by concern for others.
That's fine; I disagree, but I'm not interested in litigating it here. But your original parent actually was arguing along those lines, cf. "rat poison" et cetera, so perhaps you'd like to retroactively not reject that as "not a reasonable argument".