Warning crackpot theory coming up Ok with that out of the way here's how my theory goes. The whole source of this stems from the black garden and what it implies. The black garden was made as sort of a bastion of life (don't have exact citations but it comes from a Rasputin grimmore card) and took 7 other star systems to build (again I don't have the exact grimmore card but it's one of the Rasputin ones). This was made by humans which we will associate with the traveller. Perhaps destroying said star systems to build this bastion of life endangered other life, increasing the entropy of the universe. These star systems could have very well been the homes or part of the empires of the fallen, the hive, the cabal, and/or the vex. This would imply that the traveller has the mindset that the universe will end and entropy will dominate no matter what and it's worth it to save one species or form of life for certain. The traveller obviously allocated a lot of resources to this bastion of life, this black garden. Now think of another inverse idea which still seeks to keep life in the universe, perhaps "the darkness" has these ideals. It is very well possible that the darkness has a mentality of "we all go together" and wants to work with many species or forms of life to find a solution to entropy. If this is true, it's not hard to see why these two sides would get into conflict. This would also explain why some of the factions are seemingly hell bent on killing us with no logical reason other than them being presented to us as the personification of evil, as well as the dialogue by a dreg calling a player that snuck up on them "the darkness". Going back to the point, the darkness nor the traveller are fundamentally good or evil. This whole war is simply a conflict on the means of preserving life in the universe. This is a theory and is half based on speculation but it does explain a lot about the events of the game and gives the darkness a motivation to destroy humanity and the traveller. I've created a forum of this theory in particular https://www.bungie.net/en/Forum/Post/94358514/0/0