How did Major Marchbanks die in the Ruby and the Smoke?

In "The Ruby in the Smoke," Major Marchbanks dies as a result of being shot. He is attacked by the antagonist, who is determined to silence him due to his knowledge of the dangerous secrets surrounding the ruby and its connections to the sinister events in the story. His death serves as a pivotal moment that propels the protagonist, Sally Lockhart, deeper into the mystery she is trying to unravel.