if freed slaves owe their redeemer (jews owe HaShem) how do we look on america freeing slaves
If we look at bnei yisroel as "free" then we will not understand the true nature of yetzias mitzrayim. We were not "freed" in a complete sense -- we are bnei chorin in that we serve a different master. This sums up OUR responsibility as avdei hashem.
You can look at the Emancipation of American slaves then, in a parallel light. Someone who is not a slave to a human master still owes the institution that emancipated him -- as citizens, freed slaves have to follow the laws of the overarching community placed there by the same power that freed them, the government.