Stephen Crane's word choice does indeed create a frightful and intense mood and atmosphere for readers in this story. Crane's dark and foreboding word choice starts right from the opening sentence.

The dark uniforms of the men were so coated . . .

Notice the word "dark." We are not being told about soldiers wearing bright, clean, and crisp uniforms with shiny brass metal all over them. They are dark, and readers intuitively know that bad things happen in dark places. A few sentences later, Crane provides readers with a description of lightning. It is described as "monstrous," and that word does a nice job of deepening the frightening mood.

When a piece was fired, a red streak as round as a log flashed low in the heavens, like a monstrous bolt of lightning.

By the third paragraph, Crane is giving fairly graphic descriptions of battlefield death. We are told about a horse making a "convulsive leap of death" and a rider with a "crooked arm." In a fairly overt way to sell readers the frightening mood, Crane even uses the word "frightful" to describe a part of the battle happening on the hill.

The battery on the hill presently engaged in a frightful duel.

Moments later, Crane describes soldiers dragging another soldier's "torn body" away from the danger. Crane sells readers a frightening mood by not shying away from graphic battlefield details. He doesn't romanticize any of it. He makes war scary.