That's not actually true. The Fallen and Cabal aren't the Darkness. The Vex were originally from another Dimension but brought to ours by the Hive. This caused them to become corrupted and start to worship part of the Darkness however they still fought the Hive. The reason the Fallen attack us is because they used to have the Traveller by their side but then the Hive destroyed their homeworld and they were abandoned. They followed the Traveller and found us. Now they're trying to get the Traveller back. The Cabal are just world conqurers and are trying to take over our system.