Unfortunately, the one thing that is well documented about David Coornwinder is his demise.  Privately we know that he was married for 21 years, had 12 children of which three still lived at the time of his death in 1623. Professionally we know that he was secretaris of Berkel from 1612-1620, but his retirement as secretaris coincides approximately with the beheading of Johan van Oldenbarneveldt which ultimately was also David's undoing.
 
Oldenbarneveldt was a good friend of David Coornwinder but Oldenbarneveldt had gotten caught up on the side of the Remonstrants.  Remonstrants were Dutch Protestants, who maintained the views associated with Arminius who wrote the Five Articles of Remonstrance.  Their adversaries, the Gomarists were known as the Counter-Remonstrants. Although the states-general issued an edict tolerating both parties and forbidding further dispute, the conflict continued, and the Remonstrants were assailed both by personal enemies and by the political weapons of Prins Maurits, who executed and imprisoned their leaders for holding republican views. 
 
In addition to being a friend to Oldenbarneveldt, David Coornwinder was also brother-in-law to Bernadus Dwinglo who had married his sister Catherina Coornwinder.  Dwinglo was an outspoken Remonstrant clergyman. Somehow this connection to a religious brother-in-law and an executed friend got David Coornwinder to organize a plot to assassinate Prince Maurits.  
 
The story, as reported by John Lothrop Motley in "The Life of John Barneveld, 1619-23" goes as follows: The widow of Oldenbarneveldt, Maria van Utrecht had two sons, both in the prime of life.  The eldest, Reinier, Lord of Groeneveld, was an easy-tempered, genial, kindly gentleman who was easily impressed by others, and apt to be led by stronger natures than his own.   The younger son William, Lord of Stoutenburg, was of a far different mould. After the death of his father he was reduced a blow from luxury to beggary and obscurity. He was convinced that his personal grievances were closely entwined with the welfare of the Commonwealth, and he had sworn to avenge the death of his father.
 
Lord of Stoutenburg had around him a set of daring and desperate men to whom he had from time to time half confided his designs.  Including David Coornwinder, a friend of his father and a man of immense corpulence, and Van Dijk as lean and hungry as Coornwinder was fat.
 
David Coornwinder and Van Dijk in the last days of December 1622 drew up a scheme on paper.  The document began with a violent invective against the crimes and tyranny of the Stadholder, demonstrated the necessity of a general change in the government, and of getting rid of Maurits as an indispensable preliminary.   The Prince was in the daily habit of driving, unattended by his bodyguard, to Rijswijk, about two miles from Den Haag.  It would not be difficult for a determined band of men to kill him and beat down any defense.
 
The scheme, thus mapped out, was submitted to Stoutenburg, who gave it his approval, the document was then burnt.  It was estimated that twenty men would be needed for the job, and that to pay them handsomely would require about 6000 guilders.   But 6000 guilders was an immense sum to raise, and Stoutenburg was essentially a beggar.  The other son of Oldenbarneveldt, the Lord of Groeneveld, on the other hand, had certain means.  Many anonymous letters were sent to the Lord of Groeneveld, calling on him to strike for vengeance on the murderer of his father, and for the redemption of his native land and the Remonstrant religion from foul oppression. 
 
At last yielding to the persuasions and threats of his fierce younger brother he agreed to endorse certain bills that David Coornwinder negotiated.  David found no difficulty in discounting Groeneveld's bills, and the necessary capital was thus raised for the enterprise.  Van Dijk, the lean and hungry conspirator, now occupied himself vigorously in engaging the assassins, while Coornwinder remained the treasurer.  
 
Unfortunately, the henchmen that Van Dijk had hired talked too much in a bar in The Hague and ultimately they were brought before the Prins and told the story so far they knew it and showed the money, 300 florins apiece, which they had already received.   Maurits hesitated not an instant.  It was evident that a dark conspiracy was afoot.   He took instant measures to take possession of the two taverns where the plot had been hashed, and to arrest all the strangers found in them.  Van Dijk realized that the plot have been uncovered and immediately went to Stoutenburg.  Both men decided to escape. Van Dijk escaped through the stables and across the fields in the direction of Leiden.  After skulking about for a week however and making very little progress, he was arrested at Hazerswoude, having broken through the ice while attempting to skate across the inundated and frozen pastures in that region.
 
David Coornwinder, coming to visit Stoutenburg soon after Van Dijk had left, was informed of the discovery of the plot and did his best to escape, but was arrested within a few days.   When Lord of Groeneveld learned that the plot had been discovered, he too tried to escape, but was captured.   On the 19th February, within less than a fortnight from the explosion of the conspiracy, the eldest son of Oldenbarneveldt was lodged in the Gevangen Poort of Den Haag with the youngest son on the run.
 
The awful news struck the widow of Oldenbarneveldt as with a thunderbolt.  Both her sons were proclaimed as murderers and a price put upon their heads.  The trials of the prisoners were rapidly conducted.  Van Dijk, cruelly tortured, confessed on the rack all the details of the conspiracy.  Groeneveld was not tortured.  His answers to the interrogatories were so vague as to excite amazement at his general ignorance of the foul transaction or at the feebleness of his memory.  That it was he, who had furnished funds for the proposed murder and mutiny, knowing the purpose to which they were to be applied, was fully avowed by him.
 
On the 28th May, Groeneveld, Coornwinder, and Van Dijk were notified that they were to appear next day in the courthouse to hear their sentence, which would immediately afterwards be executed. Groeneveld was executed with a single blow that severed his head from his body.  Van Dijk and Coornwinder were executed immediately afterwards but they were quartered and their heads exposed on stakes.  What was left of  David Coornwinder was buried  in secrecy  in an orchard in Nootdorp.  

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