Neptune's mass is 1,0243 • 1035 µg.
There is not one single thing causing death, but what would cause death fastest would be to make the brain and/or heart stop working, or stop them working together by seperating them. But if some of the other organs stop working, it can also cause death. And if the blood is unable to supply the cells with oxygen, perhaps because of cold or CO, or something else binding to the blood cells.