He had a choice during the Collapse: fight the Darkness and die like his fellow Warminds, and leave humanity to be killed; or hide, waiting out the storm until he had an opening to attack, and save some of humanity. Part of his master plan to defeat the Darkness and save humanity (Midnight Exigent) was to cripple the Traveler, despite the fact that it wasn't going to leave us. Doing so released the Ghosts and a blast of Light that drove the Darkness back.