The Southerners were the people who owned slaves prior to the American Civil War. Slaves were not held in the Northern states.
This is a very simplistic view of the civil war, to say that slavery was the main issue. IT became the issue after the Emancipation proclamation in 1863. But the war started over disagreements over Federal vs State authority (Yankess called the war "The War of the Rebellion"). The North was losing the war in a major way the first two years, there were major anti-war riots in large Northern cities, esp NYC. Now as a separate movement, the abolitionist movement was gaining ground, and inflaming passions. Abe Lincoln took note of it, and issued the Emancipation Proclamitation, bringing that issue to bear on the war effort, in fact, in my opinion to get some support for the failing war effort. It worked, the war now had a moral justification, and war support in the North increased.
So to your question: Which side would you have entered on: Federalism, or the side of States being a confederation? In the end, would you have changed your mind?
You may want to look that the controversy of the book Uncle Tom's Cabin and its effect on the war, and its effect on President Lincoln.
In the end, the Yankees won, for better or worse. Slavery would have went away, in my opinion, even without the war, due to economic reasons mainly, but and partially due to the rising abolitionist movement coming from the Wesley movement sweeping the South. My family had a few slaves, which were freed at the end of the war, due to the war. They went from being a part of the family to economic hardship being cut off. Such happens when hard times come, and harder feelings split families. It is a shame. From what I can read in family history (little is available), folks then wore chips on their shoulder for about three generations, and in fact in politics, may still have very firm beliefs about the nature of things.
So when you write about what side you would fight on, we tend to look at things as we want to see the issues, which may or may not be as it was; and we see things being simple issues of right and wrong, when in fact there were other issues that were the causes that fueled the War.
War is not a nice thing to choose which side to fight, as a lot of innocents suffer, and are bloody for years.
Think this out carefully. Have fun with your essay.
Slavery really wasn't the reason for the Civil War. The primary concern was "states' rights". Did the federal government have supremacy over the affairs of individual states and if so, on what questions and how much.
Here are some sites that be explain more completely.