Interfering with Rational Autonomy

To Kant, treating a human as a means to advance one's own end is the worst moral wrong one could do. He believes this because he sees human rationality as an intrinsic good, and therefore the ability to guide's one's own life, with a free will and complete autonomy is morally good. However, is this always true? In certain situations it may seem strange or awkward to follow this imperative, and we would sacrifice a lot more than what we would gain.

Suppose a serial killer only murders people with names that begin with the letter A (you do not fit in this category). Your roommate Adam is hiding in the closet and the serial killer is knocking on your door, asking if Adam is home. If you tell him the truth he will barge into the room and look for Adam. If you lie to him there is sufficient reason to believe he will walk away to search for your roommate somewhere else. Under Kantian view, you are obligated to tell the serial killer the truth, because doing so otherwise would be to manipulate his rational autonomy (assuming he is rational).

One may point out that since the serial killer's autonomy would guide him to obstruct someone else's (Adam) autonomy more severely you are allowed to lie to the serial killer. However on a pure Kantian view this argument may not hold because the simple action of obstructing someone else's autonomy constitutes a violation of their rights, and moral restrictions are placed, not allowing the actor to move any further regardless of the consequences.

Another option would be to completely redefine who has a rational autonomy and somehow exclude the serial killer, or discount him as having autonomy because he wishes to interfere with the autonomy of others. Now, since he does not count as having an authentic autonomy the actor is permitted to lie to him. However this seems somehow strange, because it gives the impression that we can forsake our humanity. Normally when we invoke the idea of what makes us human we think of many things, with rights being one of them. Rights are given to us as birthright and only in rare circumstances can we give them up. This then brings up the question can we give up our right not to be manipulated? However, murder and being lied to both constitute as forms of manipulation against our will, so how can they be differentiated?