As a threat to the republic. Abolitionists believed that slavery was incompatible with the principles of democracy and individual freedom. They argued that the slave system created a dangerous aristocracy that was increasingly at odds with the interests of the common people, and that the presence of slavery in the United States made the country a mockery of its own stated ideals.
As an obstacle to economic progress. Abolitionists argued that slavery was an inefficient economic system that held back the South and the country as a whole. They pointed to the fact that the slave states were economically less developed than the free states, and they attributed this difference to the institution of slavery.
As a source of social conflict. Abolitionists believed that slavery was a major source of tension between the North and the South, and they feared that it would eventually lead to civil war. They argued that slavery was a dangerous relic of the past that needed to be replaced with a more just and humane system.