Recall the importance of the substance Haliperidol. Haliperidol is used by Gerard to keep Mike from inhabiting his body -- it is the 'cure' for his 'mental illness'. Apparently Haliperidol prevents spirit possession. Windom Earle also used Haliperidol after killing Caroline, and it is suspected that he did so to fake his own insanity. But I think that he too was actually using Haliperidol to avoid possession. When Earle first came to TP as part of Project Bluebook, the WL spirits recognised that his heart was pure (although there was no way of knowing what was actually in his heart), and so they originally tried to possess Earle and use Earle to find out about the BL. Earle was then being inhabited from time to time by a WL spirit. This involvement gave Earle a great deal of knowledge about the Lodges, but he decided that he wanted to enter the BL for himself in order to gain its power. To stop the WL spirits from entering his body he began to use Haliperidol. It looked to others as though the Haliperidol was causing him to act insane, but in fact the Haliperidol was simply allowing his natural evil and insanity to express itself. In fact, Earle is still using Haliperidol to this day, as implied by the fact that he injects the Major and Leo with it. After their failure with Earle, the WL spirits began to focus on the other member of the Bluebook team with a pure heart -- the Major.