This is, of course, a matter of personal opinion. My own opinion is that it should be legal for a physician to do this, but only if there are strong safeguards in place to prevent abuse.
My main reason for saying that physician-aided suicide should be legal is that I believe that people should have the opportunity to choose how they are going to end their lives. I strongly believe that people who are faced with very difficult ends to their lives should be able to go out on their own terms. For example, I would argue that a person who faces certain death in a year, and who is guaranteed to experience terrible pain and very low quality of life in that year should be able to choose to die sooner at a time of their own choosing. I think that people should not be forced to endure pain and suffering with no hope of recovery.
However, I would want to make sure that there were very strong safeguards to prevent elderly people from being pushed to end their lives. It is possible that some people would try to influence elders in their families to end their lives through physician-aided suicide so that the end-of-life health care costs would not eat into inheritances or pose a financial burden to the family. It is possible that elders might feel that they had to end their lives so that they would not force their families to care for them as their health declined. It would be important to require procedures that would drastically reduce the risk of these sorts of abuses.
There are those who argue that human life is sacred and that people should not have the right to choose when they die. There are also those who argue that doctors are supposed to preserve life, not end it. I would counter by saying that human beings deserve to die with dignity and that doctors are supposed to help us optimize our own experience of our lives. For these reasons, I believe that physician-assisted suicide should be legal.