I think it should be 100% up to the person whether they want to end their life or not (if they have terminal illness). If the person is unable to decide (mentally ill, coma, etc), then their next of kin should decide, and in the off chance that there are no/no contactable next of kin, then a doctor should decide.

Though, I'm against involuntary euthanasia, such as sentencing criminals to death. Life is a human right and should be denied to no one for any reason whatsoever, and I think that is all that needs to be said about that!