It seems your are right about going back to Germany
DEPOT SAB SOURCE BNS TYPE LEER VOLUME_NO 1/8/41 SYSTEM 01 REFERENCE 1418 PART 1 DESCRIPTION HGF SCHILLER. REPATRIATED 16/5/1919. STARTING 1914 ENDING 1916
That record description is from NAAIRS Repatriation could be voluntary. Depends on why he was in South Africa. You will have to get a copy from the archive.
Find the address here
Hamburg, Germany, Deaths, 1874-1950 are available on-line records for Hamburg.