Suicide is illegal, and should be, because society cannot be in the position of encouraging that, as a matter of principle. (However, obviously, assuming the attempt is successful, the law cannot prosecute a corpse.)
Assisted suicide is another thing. As a matter of principle, we cannot allow someone to do that, because you always have the possibility that the assistant somehow encouraged or even suggested it; and that can be a line too fine for the legal system to discern. Plus, the medical profession absolutely cannot be in the business of ending life; its task is to preserve life, and to do otherwise would utterly destroy the integrity of the profession.
That being said, any individual has the privilege of refusing medical aid. Even if that results in death, that is his privilege.