Way to uppity, underestimated his enemy, and left his heavy weaponry behind.
Had he had a shred of respect for his enemy and traveled at an appropriate speed so his bigger toys could have been used may have been a much different story.
Custer left the Gatling Guns behind for a reason -- not necessarily a bad one. They traveled in caissons which were heavy & clumsy. There was more than one incident when trying to drag them uphill the caisson would break free, roll back down the hill, bowling over cavalry horses, soldiers and supply wagons.
Also, the Gatling would not necessarily have been as effective as most people imagine given the terrain on the battlefield. It was a terrain broken by hills and coulees, and the Gatling is a "line of sight" weapon just like a rifle. Interestingly the Indians' bow & arrow was here, an advantage, since they could be launched in masse on a ballistic trajectory over a protective hill, to rain down on unprotected soldiers, who would be in no position to return fire.
Ironically, had Custer brought the Gatlings it may very well have saved the expedition -- but not in the way you mean. Had Custer been delayed by taking the Gatling Guns and been able to meet up with General Terry's column, they might -- MIGHT -- have been able to prevail.
Custer understood his enemy pretty well, all things considered. What he
didn't understand, nor could any one with him save his Crow Indian guides, was the singular and unique fact that the Sioux Indians were
expecting and prepared for an attack, and
well prepared for it. This undercut the
normally effective army strategy Custer was employing at the Little Bighorn. It was one he'd used at Washita very successfully.
As for Custer being "uppity," I suppose one has to admit that given what his own behaviour throughout his career betrays, yet it would be a mistake to believe Custer had the market of "uppitiness" in the U.S. Army cornered. Plenty of other officers -- especially so the younger ones who'd earned their reps and their ranks in the front lines of the Civil War were the same. Custer was just so much better at it.......
Gave up the high ground and lacked fire superiority. Pwnd for sure.
No, actually Custer
TOOK the high ground, atop what is called "Battle Ridge," and especially on a hill at the end now called "Custer Hill" (there's a memorial there now). The Indians called this "Seizing the Sky," and it has both offensive and defensive purposes -- though for Custer at the time it was purely defensive.
His men did lack fire superiority; the single shot Springfields they had outranged the Indians' Winchesters but this was irrelevant; the Winchesters repeating firepower was pretty decisive -- and historians believed there were more Winchester-equiped Indian warriors there than troopers under Custer.
