Why do "big" countries take over "little" countries and create lots of problems?

Today was one of the most depressing days that I have had. After my first class, which is depressing in itself, I went to my literacy and cultural diversity class. We just read this book by Earnest Gaines call "A Lesson Before Dying." A brief synopsis of it is a man in the south is arrested and sentenced to death. His cousin, who is really the main character, is a teacher and is supposed to "turn him into a man" before he is put to death. Well, we are spending a few weeks talking about issues that are raised in this book. Today we talked about race and the legal system. Starting on just a base level with cops in the community, I observed that in my community a white person does not really, ever, get pulled over or arrested. That is not to say that it never happens, obviously it does. But it is so much more likely for a Mexican or black person to get into trouble with the law. And to be honest, in my community it is almost all petty crimes. Let me pose a situation to better explain what I am angry about. If a white man is caught with under an ounce of marijuana, he will get a slap on the wrist and, at most, a citation. If a black man gets caught with the same thing, he gets thrown in jail for at least 24 hours. I am not making this up, and it is not entirely a hypothetical situation. I have known people in both. Also, did you know that to have possession of crack is seen as a worse crime, meaning it has a stronger punishment, than possession of cocaine? They are both narcotics, but one is the glamorous Hollywood drug, and one is the street drug. Then we got into the topic of Capital punishment. I don't remember the exact statistics on this, but out of the people that are on death row, the majority, by far, are black.
Right after this class I go to the high school where I tutor. I was a little late due to the intense conversation that we were having in my previous class, and when I walked in to the classroom the first thing I saw (on tv) was a small, starving child of the Sudan. We watched this horrific video about the lost boys of the Sudan that outlined their journey from all over the Sudan, to Ethiopia, then to Kenya, and then, but only for some, to the United States. I choked back tears as I watched video footage of small children with arms as skinny as my fingers taking care of even smaller children. 27,000 young boys started this on this journey, that was a flea for their lives, only 13,000 made it to Kenya.
For anyone who does not fully know the story of the lost boys, I encourage you to Wikipedia it. I can only give a brief outline. The Sudan was part of the British Empire. When they were finally encouraged by the rest of the world to grant freedom to this country, they managed to turn the North and the South against each other, and started a feud between the Christians and the Muslims. I believe it was the North who decided to kill all young men and boys in the South. Thus the journey to freedom of the Lost Boys.
After this class, I went to my British Literature class and learned about Ireland, which had a similar outcome. The North and South were separated and warred. However, this lasted much shorter than the war in the Sudan. So, I will not mention much about it here.
I just sometimes can't believe the horror of what happens in the world and what people are capable of, especially people in positions of power.