Custer might have been a glory hound and in need of something spectacular to shine his tarnished image, but he had experience in fighting Indians. He had no idea of the sheer numbers and did not expect them to stand and fight. He was more worried about them getting away and scattering than defeating his troops. Spliting his forces, in his mind, was a sound move to cover all avenues of escape. Sheridan even said that if Custer had kept the 7th intact, he should have been able to defeat them.
This is Custer's only excuse. Apparently the news that the Indians ALREADY had "stood and fought" against Crook had not yet reached Terry . . . or Custer.