Why are some soldiers mean?

I have a lot of respect for U.S. soldiers, so don't get the wrong idea by me asking this question. But, there are videos on YouTube and other websites of soldiers being really mean to Iraqi kids. For example, they were telling the kids that both them and their country smelt like a**. And most people have probably heard of the event where the one soldier threw the puppy off the cliff. So why are some soldiers like that? I know that not all are like that because I've also seen videos of soldiers that hand out soccer to the kids in Iraq. Can someone help me answer this question?