Men higher in rank than he, generals, spoke their discontent openly.

Why would not McClellan attack?
He had claimed that the rebels had two hundred thousand men at the Seven Days, when it was well known that half that figure or less was their true number.

Why should he persist in seeing the enemy double, and even if Lee did have fifty thousand men on the other side of the Antietam, instead of the twenty thousand the scouts assigned to him, the Army of the Potomac could defeat him before Jackson came up.
But McClellan was overcome by caution.

In spite of everything he doubled or tripled the numbers of the enemy.

Personally brave beyond dispute, he feared for his army.