At this time, there has been no information released on the subject of why the Vex want to kill us. Of course, I am unclear on how Destiny will address time travel. Maybe the Vex can never succeed in their crusade, due to the fact that the Vex domination of our solar system would create paradox in which the Vex that traversed time could never have existed due to the fact that we can no longer do whatever it is we do to make them want to hunt us. A forum member by the name of "IlluminatiChild" suggested to me that "The darkness is attacking the timeline of the Vex." This post got me thinking. It is possible that the Vex are a group of parasites whose universe is on the brink of collapse, so they jump from reality to reality, they live in one universe until either that universe ends or the darkness catches up with them. Perhaps the Vex themselves are what drew the darkness to our reality, in their constant struggle to escape. But what could the Vex have done to incur the wrath of the darkness in such a way to cause it to traverse time to hunt them? Of course this is all speculation.