Why hindu mythological stories are contradictory to each other in its theme and morale?

WHEN GOD HIMSELF MAKES MISTAKES- SHIVA BEHEADS HIS SON LORD GANESHA ..LORD KRISHNA HA LEAD PANDAVAS TO VICTORY BUT HE COULD NOT LEAD HIS OWN YADAVA SENA AT THE END. KRISHNA SAYS HE IS RESPONSIBLE FOR EVERY THING .EVERY LIFE ON EARTH TAKES REFUGE IN HIM.WHEN HE IS SUPREME WHY HE HAD TO FIGHT WITH KAMSA JARASANDA FOR HIS SURVIVAL