King disliked capitalism because he felt like capitalism mistreats the poor:

However, the factor that most powerfully fueled King's opposition to capitalism is the imperative of his biblical faith to bridge the gulf between abject poverty and superfluous wealth. In this sense he considered capitalism an insult to his faith. King's ethics are firmly in the tradition of radical biblical prophets like Amos, Micah, and Isaiah, who together proclaimed that everyone, including the rich and the powerful, were to be governed by ethical principles that included mishpat (foundational egalitarian justice), sadiqah (justice put into action), hesed (steadfast love; in politics, civility at the least) and emet(truthfulness, in public and in private). The political implications of this ethical constellation are reflected in this proclamation by the prophet Isaiah: "A throne shall be established in hesed (steadfast love)... and on it shall sit in emet (truthfulness) a ruler who seeks mishpat (egalitarian justice) and is swift to do sadiqah (put justice into action)" (Isaiah 16:5). From what we know of King, he was draped with the mantle of these prophetic ethics which, by definition, are fundamentally opposed to the anti-biblical foundational capitalist ethics of greed and dog-eat-dog self-dealing.