Euthanasia, as abortion, should be be a personnal decision and guaranteed under the law. If a certain person is found to be of sound mind, then everybody else has to live with that person´s decision.
And if we are talking of people suffering from severe mental disabilities, that decision should be left to a medical board, and not the family. Usually a family´s judgement is often clouded by emotions. Many times they decide based on their personnal feelings and religious beliefs, sacrificing any possiblw wishes and suffering of that person.
The hospitals are packed with people on life support, or in coma, suffering every single day because their families won´t pull the plug. A person like that has no quality of life at all, they pass their days wishing for death to pay them a swift visit.
I know that if it was me, i would rather die quicly than spend years and years stuck to a hospital bed, with no hope of getting better.
If that person can make that decision on herself, then fine. If not, because they are too sick or in coma, there should be a medical board to judge the possibilities of recovery. And if there are none, they should pull the plug and turn off life support.
Euthanasia is both moraly desirable and usefull. It prevents people from suffering needlessly, and at the same time it saves precious hospital resources.

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"Quincitilius Varus, give me back my legions!"
Emperor Augustus of Rome.