But why? Towards the end of Modern Warfare 2 we see General Shepherd turn traitor on TF141, setting his Shadow Company onto the British soldiers. Because he wants to be a hero and fulfill his narcissism he orders Graves to kill Price, Gaz, Soap, and Ghost.
However, after his 30,000 marines were killed in the capital city in the Middle East by the nuclear bomb under the order of Vladimir Makarov, Shepherd had become corrupted and ruthless as he swore vengeance toward the CIA's most wanted criminal.