Why is the definition of and punishment for other various forms of homicide left to the States?
Try the 10th Amendment: “The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.”
No government has legitimate power to alienate unalienable rights. All they have are duties in this most important regard.
In fact, if you actually understand what the word unalienable means, you realize that the individual also doesn't even have any legitimate right to alienate innocent life, even if it is his own.
unalienable: incapable of being repudiated or transferred to another
Why? Because it really isn't his own. It is only lent to him by the One Who created him.
To reiterate: Legitimate power to alienate the God-given, unalienable right to life does not reside in the United States, nor in the several states, nor in the people.
And so, you're just wrong. The Tenth Amendment, read according to the clear meaning of its words, does absolutely nothing to bolster the Ford, McCain, Paul, Romney, Palin idea that states can alienate the right to life if they want to.