It wasn't "Custer's Last Stand", really. It was Sitting Bull's (and Crazy Horse's) Ass-kicking of the 7th Cavalry. Custer just happened to be one of the egomaniacal leaders on the fateful day (and the days leading up to it). There's a great deal of 20/20 hindsight in all historical military study, but that's how we learn to not repeat mistakes. Unfortunately for Custer, he didn't learn from mistakes so much as think he was immune to paying the consequences of them. So were his co-leaders, Benteen, Reno, et al.
Their personal relationships and antagonistic feeling for one another played a huge role in the slaughter. Worst of all was the abysmal level of communication between them and the assumptions they made. Add to that, the sheer number of Lakota, Sioux, Cheyenne, and other tribes they fought against, and the skill of Sitting Bull and Crazy Horse, and you get what we know as a massacre.
Amazing stuff. Must visit the site someday and walk the routes leading to and around the battles those two days.