If I want to sell a poisonous wine to an adult that understands the risks and still wants it, then it's none of your business.

So if you want to kill a mentally ill person with their consent, that's also none of our business?

Like was this guy [1] right in what he did and his punishment was unfair?

[1] https://en.wikipedia.org/wiki/Armin_Meiwes

Well that debate escalated very quickly.