You appear to be using the word “freedom” to refer to something other than the ability to act without constraint. The phrase “My freedom ends where yours begins” (and its many variations and false attributions to various famous figures) is one of moral considerations and generally used to argue for what the world ought to be, but it is not and never has been a fundamental — i.e. even when the law falls silent — truth of the reality we actually live in.
The same is true for selling poisonous baby milk or wine.
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?