So what if he graduated last in his class at West Point and had the most demerits of any graduate. He was the best fighting cavalry officer the North had.
He didn’t exactly rise through the ranks as he flew through them. He was promoted 5 ranks in one go.
Some accounts have this unusual promotion happening by mistake.
But Custer took advantage of it, and led his new force in an aggressive attack (in violation of his orders) against Jeb Stuarts cavalry at Gettysburg.
His attack, in which he led at least 3 charges stopped the reb horses from taking the Union forces in the rear. He was also very outnumbered in that fight.
Never said he was a bad commander. He only died at bighorn because he split his forces up again. Had he kept them together he would have been able to retreat in good order.
Had he done a bit more reconnaisance, he would have discovered that the game had changed. Rather then the three prongs, he would have got everybody together. He may still have been attacked by the natives, but it wouldn’t have been a massacre. If they waited until he got everyone together, it would have been a rout.
One of the reasons he lost at Bighorn is because of his streak of success. What he had done had always worked, and he assumed that this time would be the same as all the last.