What seemed to change about Custer over time was that he was no longer a soldier fighting for his country, but a man looking for a comfortable retirement plan. As time went on, he sought success on the battlefield more for what it might bring to him personally after the fact: positive press, money, a lucrative career after the military. He didn't consider what he was doing necessarily in the course of following orders, but what a win would mean for him. He went so far as to bring reporters along so they could write about his daring and bravery. Once the news became critical, he banned the press.
Custer himself admitted that he didn't understand much; he wasn't well-rounded. He believed he could have just as easily fought for the Confederate cause. He did what he was told. He knew the Black Hills would not give up much in gold, but he wrote the report that was expected, even though he was conflicted. "The newspapers would make him famous again." He was complicit because there was something he wanted when it was all over. I would have more respect for him if he had just walked away.