Balderdash. Lee and Grant fought to win, and would do whatever it took to achieve their objective because winning as what was important to them. McClellan fought not to lose, because his reputation mattered more to him than victory did.
McClellen ran, as the Democratic President candidate, on a plank that called for a negotiated end to the war. He did not believe that crushing the Confederacy was the way to end the war. I agree, he also lacked the killer instinct that proved necessary in the end to win.