“No, federal laws against murder are not invalid, because they only apply to federal personnel and federal property under Article I Section 8.”
It doesn’t matter to whom they apply. Federal personnel are (or should be) US citizens, protected by the Constitution. If the Constitution forbids the Congress to pass laws against murder, then any murder laws it passes are unconstitutional.
I think you're unclear on the underlying principle here.
The Constitution does not forbid the Congress to pass laws against murder, it forbids Congress to pass laws outside of Congressional jurisdiction, including murder laws.
In the same manner as the Congress can't pass laws to forbid murder in China, it can't pass laws to forbid murder in downtown Tewksbury, Massachusetts. (Unless it involves federal employees, soldiers, or some other nexus to federal jurisdiction.)
Notice in 18 USC 1111 et seq, for example, that the law covers murder OF US citizens BY US citizens on foreign soil, for example, but NOT murder of US citizens by foreign citizens on foreign soil.
Does that help clarify the issue?