Romans 3:15 - John Gill's Exposition of the Entire Bible

Their feet are swift to shed blood. The sins of the heart and mouth are before described, and now the sins of action are taken notice of; for "the feet" are the instruments of motion and action: and when these are said to be "swift to shed blood", it denotes the readiness and eagerness of men, to murder innocent creatures; which shows the dreadful malice and hatred that is in them. The words are cited from Isaiah 59:7, and seem to point at the times of Manasseh, who shed so much innocent blood, as to fill Jerusalem with it from one end to the other.