Hassan is clearly a character who does great things in The Kite Runner by Khaled Hosseini. The two incidents you mention are two major examples of Hassan's sacrificial acts of bravery, and there are many other examples of Hassan's goodness throughout all of his childhood with Amir. You can make a very convincing case that Hassan is brave, loyal, and honest, and these are all wonderful qualities.
However, your prompt rather forces you to talk about the negative things that happen because of those wonderful things. So, though you will get to celebrate Hassan's bravery in saving Amir from Assef, you also have to say what it cost him. You will get to applaud Hassan for being such an honest person, a person who is known never to lie; however, you also need to say that because of that, he was used by Amir and was forced (along with his father) to leave the only home and family he has ever known. You will get to say that Hassan was not bitter about being sent away and he bravely returned to save Baba's house out of loyalty, but you will also have to say that it costs him his life as well as his wife's--and the consequences for his son were awful, as well.
While it would be nice just to concentrate on all the fine and noble qualities which Hassan possesses and focus on all the good things he did, your assignment, as I understand it, asks you to point out the negative consequences of these things.
There is one thing that you can do which satisfies the assignment but still allows you to talk about Hassan's positive characteristics. You can talk about Hassan's lack of bitterness over any of the terrible things that happen to him. This ability to suffer without growing bitter is a true gift. His father is killed in a land mine after Amir lies to get rid of them, but Hassan does not speak one word of bitterness in the letter he writes Amir. He does not hold a grudge for any of the awful things Amir did--or allowed to be done--to him.
In fact, Amir receives a letter from Amir:
I unfolded the letter. It was written in Farsi. No dots were omitted, no crosses forgotten, no words blurred together – the handwriting was almost childlike in its neatness.
Growing up, Hassan could not read or write, he learned to do these things as an adult, affirming his natural innocence, positive determination, and good character. His good qualities may have had some negative consequences for him, but Hassan's noble character does not change because of it.
Hope that gives you a few ways to talk about the positive traits Hassan has while still meeting the expectation of the prompt.