Per friend and 4th Alabama expert Rick Allen:
Butler, Felix G.–2nd Corporal April 24, 1861. Enlisted at Marion, Ala., age 19, occupation clerk. Wounded at Manassas, Va., July 21, 1861. Discharged, disability, September 3, 1861.
So it would appear that Butler died after discharge, which may explain why he is not listed in Robert E. Smith’s Confederate Casualties at First and Second Manassas.