Assisted suicide is a form of homicide, but not all homicides are criminal. As just one example, homicides committed in self defense are not deemed criminal in any state, although state laws vary with respect to the boundaries of that particular defense.
The Fifth Amendment was adopted as an express limitation on the exercise of Federal power. I can think of no one connected with the adoption of the Fifth Amendment who thought that that amendment would serve as a limitation on the right of states to determine for themselves which homicides should be made criminal or non-criminal in their own jurisdictions.
More importantly, what I think you may be missing is that Dr. Keyes does not appear to be relying upon the Fifth Amendment or any other provision of the Constitution. Instead, he seems to be looking to the Declaration of Independence as a source of Federal power. That is what I find to be most remarkable about his analysis of this problem.