Why did Saidu say soon he will completely die and the only thing left behind would be his empty body walking with them?

If you examine the whole quote it will make more sense although I think Beah says this,

"Every time people come at us with the intention of killing us, I close my eyes and wait for death. Even thought I am still alive, I feel like each time I accept death, part of me dies. Very soon I will completely die and all that will be left is my empty body walking with you. It will be quieter than I am.”