Theories
Felix Killed the Sheriff

Warning This Section Does Contain Spoilers

As we know from season Transformation, Felix is capable of killing. He either murdered himself or ordered Aaron de Vaille murdered. The Sheriff was becoming a liability concerning the body in the woods. In Felix last letter from episode four, Felix tells us that the Sheriff would be gone by the time we read that letter. But then we get another very cryptic letter from the Sheriff in episode five. Although, we know this letter is not really from the Sheriff. Did Felix kill the Sheriff? This would be more plausible than Jasper killing the Sheriff. Jasper would not have access off campus to the Sheriff's stationary to send us this last letter from the Sheriff, but Felix would. Quieting the Sheriff would keep things under wraps about the body in the woods, for what ever reason Felix has for keeping that body a secret. If that body is the doctor and Jasper has killed her, Felix may not want this common knowledge until he can prove it.