You're still working on the assumption that dark is evil. Bungie could be switching it around to show that good and evil are a matter of perspective. The Fallen may also be on Earth because our enemies (possibly even the Traveler) had already banished us to the city. From what the lore tells us, it doesn't sound like only the Fallen caused the entire collapse. Maybe the Traveler even caused it. The lore does mention, however, that the Fallen and other enemies appeared on our former planets AFTER the collapse. They could simply be scavengers who found empty land.