Within Reason

Many people studying ethics for the first time are scared off by the first formulation of Kant’s categorical imperative: Act only on that maxim (principle) which you can at the same time will to be a universal law. You’re constrained to acting on principles that can be willed to be universal laws. But the easiest way to see that something isn’t universal is to find one counter-instance. If your principle would conflict with even one other particular person’s trying to will the same thing, it’s out of bounds.

So if I want to kill someone because I would feel a lot better if he was dead, this is pretty easily seen to conflict with his desire to do the same to me. If it doesn’t work with this one other person, it’s not going to universalize. Much more simple than going through with the whole universalizability process. (Though, of course, if it seems to pass this first test then it still must pass the universalizability test.)