Although this movie is inaccurate overall, there are some items that may be true. Certainly, he was a wild character in his youth, having played practical jokes on his fellow cadets at West Point, almost expelled several times, graduating last in his class (of 34), and often reckless in his leadership during the Civil War.
But history may have made him a scape goat of the Indian Wars. Certainly, he did his share of cruel things, but how much was he under orders? Also, there is evidence that he testified before Congress (at great risk to his commission and command) that he argued about the fairness of breaking treaties with the Indians and that if he was an Indian he would also fight rather than live on a reservation!
As a character said in the play 1776 when asked what will be said about the British about losing the Revolutionary War, the character states "history will do what it always does...it will lie." Who knows how bad a man Custer was. Certainly he wasn't the sympathetic character as portrayed by Errol Flynn and later by Ronald Reagan. But I also doubt he was completely evil as he is later portrayed.