The term I was looking for regarding American denial of involvement in Ambassador Karlov's assassination: implausible deniability.
In case you we worried that Barry had to forego a single shekel over The Abstaining: "Behind the Scenes of the U.S. Decision Not to Veto the UNSC Resolution":
"The U.S. delegation to the UN made clear to all other UNSC delegations that any resolution that failed to reflect concerns over Palestinian incitement, violence, and terrorism would be subject to a U.S. veto.""Netanyahu Summons U.S. Ambassador Over UN Vote on Settlements"!