Given that other nations still had slavery why did the abolitionists believe that the guilt for such oppression lay more heavily on the united *** states? and why did they accuse the national and free-state governments of aiding and abetting the southern states in the continuation of slavery?

The US was the 'city upon a hill': Using early Puritan ideology abolitionists saw the US as a beacon of equality and freedom which slavery spit in the face of. 

The US had been a model for political philosophy and an experiment which was mostly a success but slavery was a flaw. Abolitionists believed we could not be a beacon of freedom with the system in place. The US government refused to deal with the issue and compromises furthered the practice instead of limiting or eliminating it.