Oh my God that's beyond easy, I'm literally looking at Fall of Reach right now. He was the first S-II to die by getting killed on a Covenant ship he boarded. Originally, the team was supposed to be larger, but the only Spartans who made it besides Sam (onto the ship) were John-117 and Kelly. The book never tells what exactly killed him - it is assumed that it came from the plasma round that hit him when he saved John-117. The last time we see him, he is choosing to stay behind, as he can't even possibly hope to get off the ship because of holes in his armor. Man I could give you a detailed synopsis of that entire book, front to back. Give me something from Ghosts of Onyx or the Flood.