One of the best ways to understand an ethical theory is to understand an opposing theory…
The underlying idea behind Kantian ethics is that each human being has inherent worth. Simply because you are a human, you have worth in and of yourself. Kant’s evidence for this is simple (or, rather I can explain it in a simple way), without human beings, there would be nothing “valued” — so, since the value must come from someplace, it must be from human beings.
Further, Kant argues that human reason facilitates human autonomy. So, we can reason to what we want to accomplish in the world — i.e. we can make decisions about how to act and the overall course of our lives. Thus, we can also reason to right behavior.
Kant’s idea is that the Categorical Imperative should function as a decision rule for right action. The general idea behind the CI is that you shouldn’t act on motives you wouldn’t want to be universal law… so, in essence, you shouldn’t do what you wouldn’t want others to do….
One of the major variations on the categorical imperative is the “means / ends” formulation… which makes an important point about Kant’s view of humanity — namely, that you ought not treat humans as a means to an end. In other words, you shouldn’t use people to get what you want.
All of this boils down to a test of your motives — if your motive for action (maxim) passes the categorical imperative, your action is permissible.
For utilitarianism, you may use whatever means (act on whatever motives) are necessary to achieve an end that increases happiness. It doesn’t matter why you did the action, only that the end result is an increase in happiness.