in ch.23, why do you suppose Crane spends so much time on the rival flag bearers death?

I think Crane is bringing a sense of oneness to both sides. Just as Henry feels his own sense of pride, manhood and purpose wrapped up in his flag, so does the flag bearer in grey feel the same thing,

"He perceived this man fighting a last struggle, the struggle of one whose legs are grasped by demons. It was a ghastly battle. Over his face was the bleach of death, but set upon it was the dark and hard lines of desperate purpose. With this terrible grin of resolution he hugged his precious flag to him and was stumbling and staggering in his design to go the way that led to safety for it."

Notice that Crane's diction does not change when referring to Henry's enemy and counterpart in grey. His language is just as brutal, descriptive and haunting.