He got a big piece of shrapnel lodged in his stomach from the plane crash. Jack, the doctor, tried hard to save him but just couldn’t. In document C, Jack was determined to save Mars but he couldn’t without the proper medicine. Jack got the shrapnel out but he’s still suffering. Kate and Sawyer talk about taking him out of his misery, Sawyer decides to kill him with the gun but misses his heart and hits his lungs. According to Jack, Mars would have internal bleeding and have a painful death. They choke him out to end it. This is second degree murder because as stated in document A, they intentionally killed him. According to document C, Jack intentionally killed Mars to mercy