"He is none of us" here signifies that he is not following the ways of Muslims. "The ways and traditions of the Days of Ignorance" means here wailing, i.e., 'Alas! my lion, my moon, my support, one whose death has rendered his children orphan etc., etc.' This is a grave sin and there is a stern warning that one who does it, is in danger of losing his Faith because by doing so one, in fact, expresses resentment against Allah's Command.