Poor LTC Custer. I can't think of a single American serviceman killed in action who is so regularly belittled.
There is an old German saying, "Viel Feind', viel Ehr'", meaning "lots of enemies, lots of honor".
We Americans are a bit peculiar about "heroic" military leaders: we love to worship them, but on the other hand we also have a thing for revisionist versions of history that tear them down.
In Custer, these two tendencies slammed into each other with a vengeance.
Take the spectacular nature of the event itself, a routine round-up of some wayward tribes for return to the rez turning into a most humiliating disaster. Now on one side there is Libby Custer, devoting her quite lengthy remaining lifetime (she died in 1933) to defending and polishing her dear husband's heroic image. On the other side there are, starting immediately after the battle, many officers and politicians who need explanations and a scapegoat, preferably a dead one. Very public events such as the trial of Major Reno in 1879 did their part.
Add to that the fact that for over a century after the battle, nobody knew what really happened with the Custer battalion in the final hours. As we all know, we absolutely love historical mysteries; just mention JFK. Basically, historians and an enormous number of amateur "historians" produced 120 years of books and articles, reinforced by Hollywood, that created a largely distorted public image of the fight, because the evidence wasn't available.
Since the comprehensive archaeological excavations after the fires in the 1980s, a lot has changed. You'll have to read up on this yourself.
The picture of Custer's likely decision-making that emerges is quite complex. Mistakes were made, opportunities were missed, and not just by Custer. But as events unfolded, generic vilifications of Custer are unhistoric and pointless.