Peter thins them out
All wanted blood except the boys, who liked it as a rule, but to-night were out to greet their captain. The boys on the island vary, of course, in numbers, according as they get killed and so on; and when they seem to be growing up, which is against the rules, Peter thins them out; but at this time there were six of them, counting the twins as two.
The controversial phrase here is “thins them out”. How would you interpret it?