Several years ago a series of letters written by Captain Otto Ernest Michaelis, the Ordnance Officer for the 7th Cavalry, during the 1876 campaign were put up for sale. Michaelis remained with General Terry when Custer went to the Little Big Horn. The following is an excerpt from a letter written shortly after the battle concerning the cause of Custer's defeat.

"It was obvious. He underestimated the enemy, and made a fatal mistake in dividing his command, and thus permitting the enemy to defeat him in detail. He probably was deceived as to the distance he had to travel around the bluffs, and thus lost all the possible advantage to be gained by a simultaneous attack in front and rear. As he knew that General Terry would be on the Little Big Horn on the 26th, and as the enemy made no signal of diparture [sic] - he might have waited, sent a scout, and thus insured a united attack. Had he carried out the spirit of his instructions to proceed southward... the two commands would probably have arrived at the camp at the same time. Even if Gibbons column had reached the battleground first, we could easily have minted ourselves with our infantry and Gatling battery ... All admire Custer's dash, spirit, courage - and only criti-cize his judgement so far as Indians are concerned. Some who knew him well say he would have charged this camp with any force he might perchance have had at his disposal. This battle has proved conclusively that the Indian in defense of his Household Gods will fight with desperation - especially when he is backed by the knowledge that he outnumbers the foe 5 to one"

Not that it would've affected the outcome, another roadblock was the 7th's rifle ammo. It was copper cased, not brass. After firing, the empty swelled to the point where it wouldn't eject. The trooper had to pry the empty out with his knife. While everything said by Custer's detractors might be true, we shouldn't forget his efforts against Stuart's Confederate cavalry the third day at Gettysburg. Had Stuart prevailed, there might've been a completely different nation, and western expansion would've been problematic. Little Big Horn might have never happened.

I believe you and a few others nailed it, especially in your opening line (which I bolded)! After studying the American Civil War and in particular the battle at Gettysburg for many years and as much as I respect General Lee, this is exactly what led to his army's tremendous loss there. In my opinion, Custer had many faults but being slaughtered along with his men wasn't one of them. His ego / ambitions combined with the skill and tactics of the Sioux warrior, led to his own destruction.