No one today can morally justify slavery which the Constitution effectively recognized as a political reality by allowing 3/5's of slaves in the slave owning states to be counted for purposes of representation in the House of Representatives. However, it is unfair to judge the framers in 1787 by the standards of today. Without some compromise on the slavery issue, no union was possible. Nevertheless, this constitutional compromise and the delegation to the states of voter qualifications helped lead to the diminution in the 1790s of such political rights as free blacks had enjoyed as a result of the Revolution. The Constitution in the short run was unjust to them. Women did not gain the suffrage until 1919 and most southern blacks not until 1964-1965. The position of the most populous element, American Indians, remained perilous under the Constitution. Thomas Jefferson referred to them as 'savages' in the Declaration of Independence. Article I, section 8 gave Congress the power to regulate affairs with the Indian tribes, which might have but did not prevent discrimination against them. The Constitution was intended to serve the interests only of the immigrant community which settled the former colonies.
Therefore, while the Constitution produced justice for some and injustice for others, it also provided a framework for the peaceful achievement over time of a more just society.
In summary, the republican Constitution helped redistribute power to the established economic elites but it was a sufficiently flexible document to permit future changes which would lead to new political and economic realignments of power.
Stewart, Theresia, Virginia Stowitz, Karen Sunshine, and Christine Schultz. Reader for American Government and Politics in the New Millennium. 2nd ed. Wheaton, IL: Abigail P, 2000.