The north did not want slavery to expand because it would give the south more power in Congress due to the three-fifths clause in the Constitution. Other than that, the majority of them had no problem with slavery. Slavery kept a majority of African-Americans in the south, and that's where most northerners wanted them to stay. Let's also not forget that Lincoln supported an amendment to the Constitution to make slavery permanent, and stated many times that he did not support nor did he want equality between white and black people.
The south starting the war is also a lot of nonsense. Jefferson Davis sent delegates to Lincoln to offer to pay for all the federal property in the south, and their portion of the national debt. Lincoln refused to meet with the delegates. He then attempted to re-supply Fort Sumter knowing that the south would not allow that to happen because they wouldn't want any Union bases in their country. They were willing to let the Union troops run out of supplies and simply abandon the base in due time, but if Lincoln was going to try and keep a Union presence in the Confederacy they were going to stop him. Also, nobody was killed in the attack on Fort Sumter, and all the Union soldiers were allowed to return to the north unharmed.
It was also not one of our most bloody wars because of the south. It was Lincoln who allowed his troops to wage war on civilians, not Jefferson Davis.
Let's also not forget the words of the founding fathers:
"...Governments are instituted among men, deriving their just powers from the consent of the governed,--That whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it, and to institute new government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness." - The Declaration of Independence