People for euthanasia say that voluntary euthanasia will not lead to involuntary euthanasia. They look at things as simply black and white. In real life there would be millions of situations each year where cases would not fall clearly into either category.
Here are two:
Example 1: an elderly person in a nursing home, who can barely understand a breakfast menu, is asked to sign a form consenting to be killed. Is this voluntary or involuntary? Will they be protected by the law? How? Right now the overall prohibition on killing stands in the way. Once one signature can sign away a person's life, what can be as strong a protection as the current absolute prohibition on direct killing? Answer: nothing.
Example 2: a woman is suffering from depresssion and asks to be helped to commit suicide. One doctor sets up a practice to "help" such people. She and anyone who wants to die knows he will approve any such request. He does thousands a year for $200 each. How does the law protect people from him? Does it specify that a doctor can only approve 50 requests a year? 100? 150? If you don't think there are such doctors, just look at recent stories of doctors and nurses who are charged with murder for killing dozens or hundreds of patients.
Legalised euthanasia would most likely progress to the stage where people, at a certain point, would be expected to volunteer to be killed. Think about this: What if your veternarian said that your ill dog would be better of "put out of her misery" by being "put to sleep" and you refused to consent. What would the vet and his assistants think? What would your friends think? Ten years from now, if a doctor told you your mother's "quality of life" was not worth living for and asked you, as the closest family member, to approve a "quick, painless ending of her life" and you refused how would doctors, nurses and others, conditioned to accept euthanasia as normal and right, treat you and your mother. Or, what if the approval was sought from your mother, who was depressed by her illness? Would she have the strength to refuse what everyone in the nursing home "expected" from seriously ill elderly people?