In OTL, Antietam was more or less a bloody draw, Lee's army took 25% casualty but was able to withdraw in good order and save his army. This was largely because McClellan did not take any of the several opportunities to seize victory due to his overcautious nature. Namely, the failure to commit his reserves into battle at two crucial points of the battle allowed the Confederates to regroup after his troops had all but broken through their lines.
Let's say McClellan is slightly less cautious, and he commits one of those corp (Porter's) into the bloody lane as the Union troops breakthrough (as per OTL) and exploits it. He also commits his other corp (Franklin's) onto his right as Burnside temporarily broke through the Confederate left and allows him to exploit his victory as well.
What happens then? Does the Army of Northern Virginia ceases to exist? Does Lee limp back to Virginia with 50-60% of his army gone? What would be the aftermath of such a defeat? We discuss a lot what happens if the south wins at Antietam, but what happens with a complete union victory?