The man is about to commit harakiri:
刀を前へ廻し、腰を持ち上げ、上半身が刃先へのしかかるようにして、体に全力をこめているのが、軍服の怒った肩からわかった。
What is understood here by "pointy shoulders" (if I'm understanding 怒った肩 correctly)? How can pointy shoulders (which I assume to be part of his uniform) show that he was gathering his strength?