Balance
Jack Ruby and Lee Harvey Oswald are undoubtably meant to have a connection in Libra . We see a kindred spirit in the two in the violence the two display--Oswald to his wife, Ruby to his patron. Both are ex-military men, although Ruby appears to have been much more loyal to the U.S. Oswald and Ruby both use fake names. I think that DeLillo is trying to show the reader that Ruby is also a libra in spirit--trying to balance his darker emotions. We can see the violence that he is capable of displaying when he fights patrons or shoots Oswald ultimately, but we can see his softer side, toward his dogs and the like. Perhaps if Oswald and Ruby had met under different conditions, they might have found a kindred spirit in each other.