In chapter 2, What do you think the captain meant by "If it comes to swinging, swing all, says I" ?

Basically the Captain has had enough of whatever Black Dog had to say. THe Captain means that if Black Dog wants to fight (swing), he will oblige him,

"No!" he roared at his "old shipmate." "No and no twice, by thunder! If it comes to swinging, swing all, says I!" He and the other dog jumped up, drew their cutlasses, and began to fight. It was a brief but intense spectacle as they dueled, until Billy managed to knock Black Dog's sword out of his grip. Black Dog fled in terror, with Billy close behind him still hacking at the air just inches short of the sailor's hairy hide....