Immanuel Kant argued that we should evaluate people’s actions only in terms of the principles they choose to follow and not in terms of their emotions. Why does Kant make this claim? Do you think he is right? How can I write an essay on this topic?

First, you will need a thesis statement. The thesis statement guides the scope and direction of your essay, so it's a really important component of your paper. The thesis statement not only keeps you on track but also helps your readers understand where you're going with your argument. So, you will have to take a position in order to craft an effective thesis statement. Let's just say you decide to support Kant's argument; your thesis may look something like this:
We should evaluate people's actions based on the principles they follow rather than the emotions that inspire them, because it is only their commitment to duty that gives their actions real moral worth. 
If you decide to disagree with Kant's argument, your thesis statement might look something like this:
We should evaluate people's actions based on the consequences of their actions rather than the principles and emotions that guide them, because good consequences retain their worth regardless of their inspiration.
Ideally, a thesis statement should be placed in the first paragraph. Also, consider providing three brief examples to support your thesis statement in your first paragraph. You can then provide greater detail about those three examples in the body of your essay. 
Try beginning your paragraph with an interesting scenario for your readers: A friend of yours is fleeing from someone who wants to kill him. The killer asks you whether you've seen your friend. You lie and say that your friend has gone to the train station. Meanwhile, your friend is hiding nearby. The killer threatens to kill you later if you've given him the wrong information. You nervously nod and make way for the killer to run past you. According to Kant, your lie is morally wrong unless you can justify lying as a universal principle or maxim everyone should follow at all times. 
Let's say you disagree with Kant (please see my second thesis statement example above). You would argue that since you saved your friend's life, your lie was permissible. Good consequences retain their worth regardless of their inspiration. Let's say you lied because you felt great loyalty to your friend; he once saved your life. Or perhaps you lied because you felt that your actions were based on moral duty. Your universal principle would then be something like this: it is permissible to lie if someone's life can be saved as a result of that lie. If you live according to this principle, you can argue that you're still living according to Kant's definition of good universal principles or maxims. Incidentally, this supports the argument in the first thesis statement.
So, whether you act from emotion or principle, good consequences continue to retain their worth. In the body of your essay, provide three more scenarios that support your thesis statement (you would be using the second thesis statement example above). Feel free to tweak the thesis statements above to your specifications. Use one scenario for each paragraph.
Here's another scenario: Suppose you decide to give some money to a charitable cause. You do so because you want to impress a girlfriend/boyfriend. However, you don't divulge the reasons for your magnanimous actions. Because you decided to give a sizable donation, good things happened. Perhaps your donation finally made a new wing at the children's hospital possible. 
Of course, you weren't inspired by any moral principles when you gave the donation; your emotions were the prime inspiration. However, your actions produced good results. This example can support the second thesis statement above.
If you support the first statement I crafted above, provide different examples to support your argument. End your essay by reiterating your thesis statement and perhaps addressing some counter-arguments. For example, if you chose the first thesis statement above, briefly explain why it is wrong to rely on consequences to judge a person's moral worth or the worth of his/her actions. Remember to refrain from introducing any new material in the last paragraph. Your last paragraph should be used to strengthen your initial argument from the first paragraph.