Upon receiving Maleficent's invitation, Shere Khan declined, for socializing is antithetical to the nature of tigers. However, when he had an idea, he accepted it and entered Canterlot, now under Maleficent's rule. When she presented her plan, he was thrilled, for his hunch was right--Maleficent wants to reawaken her master, the evil of all evils, Chernabog. Scar was there merely for the credit, and the tiger tolerated it...to an extent. Even though it was Shere Khan who captured the master waterbender Katara for Maleficent, he put the credit to Scar so that their enemies could believe Katara to be dead and blame it on Scar and his forces.
The tiger did not consider, however, a certain purple unicorn who was merely there when she saw what really happened.