Natsu was the normal guy in the group, something weird happens then he acts like any normal person would. Call it weird and move on with life. He was funny, because.... well cuz he was normal. He's that kind of guy that would go, "you're crazy" etc, but he faded away. His individuality disappeared along with the normal people.
Like every kid here, we go to school. Listen to teachers rambling and grouping up with people you knew and talking. Natsu was as normal as a guy gets. Always shy when he (rarely) hugged Maya, wasn't into the whole "holding hands" thing. He was basically any normal little boy. There's just no way to explain how, if or did die.
He just faded away, blending into society along with everyone that's "normal"
