When I was looking throught the packet, this was definitely one of my favorite poems. Apart from the fact that it was only one stanza and was simple enough to understand, I also loved the eccentricity of it. It seemed like Crane just randomly decided to jot down a separate thought from what he had been writing before.
The way I interpreted it, the devils were a part of the speaker's heart. Therefore, I assumed that they were his thoughts and feelings. Since he was comparing these thoughts and feelings to little red devils, they are probably either things that he is ashamed of or feels guilty about, or things that he does not think he should be writing about, and so is trying to avoid confronting them at any cost. He also says that they are "so tiny the pen could mash them," which could mean that they really weren't significant enough to write about anyways. However, the little red devils still struggle in the ink of the pen which is apparently mass murdering them, so they have to have some kind of influence in his heart and mind.
That being said, the tone was not particularly emotional. There were no sad feelings about mashing the little devils, but there was no excitement either. He was merely pondering the fleeting thought of it as he kept spilling his heart out over the paper, devils and all. "It was strange to write in this red muck of things from my heart." Despite the fact that this was basically a genocide of tiny little red devils that came directly out of his heart, the only thing he seems to have been worried about was that it was strange to have to write through all the blood. But I think this indifference of the speaker in this poem really reinforces the idea of it being a random thought, since we generally tend to brush off these thoughts as our train of thought continues on its path, and we therefore don't get very emotionally invested in any one specific idea. That's what really stood out to me in this poem, and I think that's why I enjoyed it so much.