For Paul it all comes down to the Constitution - if the Constitution does not specifically authorize Congress to act on all of the particulars a bill contains, he votes against it. Makes for some interesting votes for sure, but also explains why he's arguably the biggest supporter of the second amendment in Congress.
But in this case, the Constitution specifically authorizes Congress to do just that. Article 1, section 8, subsection 10 (a clause Paul quite frequently overlooks)- Congress is authorized to define and punish offenses against the laws of nations. The Constitution doesn't limit what the 'punishment' is, it leaves that for congress to define as it sees fit. It does, however, in several areas indicate that treaties are to be considered high law of the land and must be honored. We have several international treaties in which we are co signatories agreeing to condemn any acts of genocide.
Like in many other cases, Paul simply likes to pick and choose from the Constitution in order to fit his view while claiming his view is defined by the Constitution.