He uses the slave codes of the time to display the slaveowners' characters in an unfavorable light:
Whatever may be the eminence of individual virtue - and I would not so far disparage humanity as to suppose that the offences which may be general where Slavery exists are universal - it is not reasonable or logical to infer that the masses of Slave-masters are better than the Law of Slavery. And since the Law itself degrades the slave to be a chattel, and submits him to their irresponsible control - with power to bind and to scourge; to usurp the fruits of another's labor; to pollute the body; and to outrage all ties of family, making marriage impossible - we must conclude that such enormities are sanctioned by Slave-masters; while the exclusion of testimony and prohibition of instruction - by supplementary law - complete the evidence of their complicity. And this conclusion must stand unquestioned just as long as the Law of Slavery exists unrepealed. Cease, then, to blazon the humanity of Slave-masters. Tell me not of the lenity with which this cruel Code is tempered to its unhappy subjects. Tell me not of the sympathy which overflows from the mansion of the master to the cabin of the slave. In vain you assert such "happy accidents" In vain you show that there are individuals who do not exert the wickedness of the law. The Barbarism still endures, solemnly, legislatively, judicially attested in the very SLAVE CODE, and proclaims constantly the character of its authors.The New York Times has the text of the speech online. I rely here on the text from the version published in 1863 as Barbarism of Slavery.