There is something bothering me. Slicer and his brother died after being stabbed with a sword, but Alphonse Elric didn't die after getting his body broken apart by Scar. How did this happen?
Edward looked so confused learning Slicer was still alive after he chopped Slicer's head. I mean he knows that his brother's body has experienced much worse damage and gotten his head chopped too and still alive.
Slicer's blood runes were destroyed, which is what killed him.
Slicer had two souls in one armor, so he had two blood runes. Ed didn't know that at first. He only knew of one blood rune in the helmet, and thought that cutting off the head would stop the body. But there was another blood rune in the body.
Al's blood rune is not in his helmet, so his head can be cut off without hurting him.
As long as the blood rune is intact, the soul survives. The rest of the armor can be severely damaged.
