Leading his troops into a slaughter while chasing a bunch of people who wanted to *leave* the US still makes no sense. That’s what got him a bad rap from history. Too bad this action all but negated the amazing record he had in the Civil War.
A few years ago I read an the book: "Custer and Crazy Horse" and in that book there is an excellent explanation.
Custer had considerable experience with fighting Indians and in virtually every case he and his men were badly out numbered. However, the Indians always used a sort of "hit and run" style of attacking. The Indians having much less fire power would always flee, in spite of their superior numbers.
That day Custer had no reason to believe the tactics would be any different, he also was not aware of the full size of the force he was up against.
That day, the Indians did not flee, they had in fact adjusted the battle plan and Custer paid the price.