You promised your friend you would keep a secret. A lie would protect him from harm, but the truth would hurt no one else. Kant says you cannot lie. The rule matters more than the result.
Deontology says some actions are wrong in themselves. You do not look at consequences. You look at the kind of action it is and whether you could want everyone to do it.
Kant's categorical imperative gives the test. First version: act only on maxims you can will as universal laws. Second version: treat humanity always as an end, never merely as a means. The framework protects people from being used. It also stops you from making exceptions for yourself. If lying is okay when it helps you, then lying becomes okay for everyone and trust collapses.
Act only according to that maxim whereby you can at the same time will that it should become a universal law. [...] Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.
— Immanuel Kant, Groundwork of the Metaphysics of Morals, Section II (1785, trans. Jonathan Bennett modernization)
Kant first gives the universal-law test. You imagine your reason for acting becoming a law everyone follows. If the world falls apart or you could not coherently want that, the maxim is forbidden. The second formulation protects persons. You may never use someone solely as a tool for your goals. The law comes from reason itself, not from what happens to follow.
A murderer knocks at your door asking where your friend is hiding. You know the friend is inside. Kant says you cannot lie. The maxim "lie to save life" cannot be universalized without destroying truthful communication. People in occupied Europe who hid Jews sometimes followed similar reasoning and refused to give false information even when it risked lives.
The murderer will kill your friend if you tell the truth. The framework says you still cannot lie. Is that the price of consistent morality, or does it show the limits of pure duty?
Quick reflection
According to Kant, why can you never treat a person merely as a means?