And thus said Shimei when he cursed, Come out, come out, thou bloody man, and thou man of Belial: The LORD hath returned upon thee all the blood of the house of Saul, in whose stead thou hast reigned; and the LORD hath delivered the kingdom into the hand of Absalom thy son: and, behold, thou art taken in thy mischief, because thou art a bloody man.
Although the phrase had been used for a number of years by Independents, it became politically significant in April 1648 during the three-day prayer meeting at Windsor Castle by the leadership of the New Model Army. The Army leadership felt deeply betrayed by the King because they thought that while they had been negotiating in good faith he had duplicitously gone behind their backs in making The Engagement with the Scots and encouraging a new civil war. At the end of the meeting the Grandees of the Army accepted that it was their duty "to call Charles Stuart, that man of blood, to an account for that blood he had shed, and mischief he had done".
Peter Gaunt, The English Civil War: The Essential Readings, Blackwell Publishing, 2000. ISBN 0-631-20809-7, ISBN 978-0-631-20809-9. Chapter 19 Charles Stuart, that man of blood. (Originally published in Journal of British Studies 16 (1977):41-61)