*** w98 12/15 p. 22 When Armed Robbers Strike ***
Some people desperately seek security by arming themselves with guns. Christians, though, take seriously the words of Jesus, who said: “Those who take the sword will perish by the sword.” (Matthew 26:52) God’s people have ‘beaten their swords into plowshares’ and do not buy guns to protect themselves from robbery or assault.—Micah 4:3.
The WTS does not elaborate too much on prohibiting purchasing guns for the purpose self defense from other humans. But the above quoted article clearly states that God's people do not buy guns for protection from other humans. If God's people do not buy guns for protection from humans then logically anyone buying a gun for such purpose is not God's people.