The option to kill always punishes the most vulnerable. Those who are wealthy and currently fly to a jurisdiction where the killing is legal will find options for themselves, while laws that prevent killing are there to protect those who would otherwise want to live if not for the system that tells them they are or would be a burden. Laws that allow killing entrench and reinforce a culture that values the intelligent and able-bodied while making the disabled and infirm disappear.
Those who have struggled with severe depression will tell you that one of the worst thoughts to haunt their minds is the one that says they would have been better off having never been born. When I treat patients with depression, it is my duty as their doctor to assure them, no matter what they may have done (and some of them have done very bad things), that the demonic voice in their head is not speaking the truth. They, like every other human being, are better off alive than dead. To make the opposite judgment on behalf of another person — most of whom, if they were allowed to grow up, would be able to have some opinion on whether they prefer being alive or dead — is taking the side of the suicidal voices against the God who created us all.