Standard treatment for acute staph infection in 1920 was lancing the abscess
and hoping the infection healed after draining out the pus. Sometimes this worked and sometimes it didn't. In a very famous and highly publicized similar situation President Calvin Coolidge's son died in the White House from a lanced toe infected with staph aureus in 1924. If the infection didn't come out when the abscess was lanced, the infection would turn inward causing sepsis
and death. I think that is what happened in the death certificate you mention. The person died from massive sepsis with the endocarditis
being a likely final cause of death.