Their feet are swift to pour out blood

Here "feet" is a synecdoche that represents the people themselves. The word "blood" is a metaphor that refers to killing people. Alternate translation: "They are in a hurry to harm and murder people" (See: rc://en/ta/man/translate/figs-synecdoche and rc://en/ta/man/translate/figs-metaphor)

Their feet

The word "their" refers to the Jews and Greeks in Romans 3:9.