Was it necessary to fight a war (Civil War) in order to end slavery?What alternatives could have avoided violence?

In short, yes because so much was at stake that the rich plantation owners would never give up their free labor; and they were truly ignorant of African people and their rich ethnic culture in Africa.

First of all, from 1650 to 1790 during the colonial times slavery was everywhere in the country. However from America's independence until the Civil War(1790 -- 1865), it only occured in the south; and it was hotly debated between the north and south. After American independence, the Northern states: New England, New York, Pennsylvania, New Jersey viewed slavery as opposing the spirit of the revolution. In the north, slaves were less common because so many white men could do jobs that poor blacks did. In fact, working class men didn't want black slaves around because they wanted the jobs, so this demand to abolish slavery was strong in the north.
In the southern states it was very different.The numbers of African Americans were very large. Half of the people in Virginia and South Carolina in 1790, were African American.
Southern whites were very ignorant and had an ingrained racial prejuduce. They believed that African Americans, if free, would ravage the countryside and be a threat to everyone. Southerners thought--or told themselves--that slavery was "race control".
The economic role the slaves played is the most critical reason that it was kept in place.The southern states' agricultural production was dependent upon slavery. For example, in Delaware, Maryland, Virginia, the Carolinas, and Georgia, all of their wealth was a result of the hard work of slaves. Actually they didn't think that it was possible for "white men" to do such hard work. These plantations produced tobacco, cotton, rice, and indigo.
The Southern states insisted on keeping slavery even after the Revolution. This is the reason for the huge fight between the free states and the slave states. Of course Lincoln was on the side to free the slaves. And the civil war began.

I think it is obvious why the war was inevitable: economics.

I don't think that violence could have been avoided.