One of the reasons alchemy was feared, as well as A.M.R.O's existence, was to curb the menace that was insannadiction. 

As a person is over whelmed by insanity and they cross the fine line and lose all humanity they remove all limiters. While we are human we can't use fully utilize alchemy (given a few exceptions such as chronos etc), however those succumbed to insanity can wreak havoc. They have no limiters.