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.