What did Henry Knox mean that the "desperate and unprincipled men" were "such a threat to every man of principle and property in New England" and how does he support this belief?

Your question seems to relate to a letter Henry Knox sent George Washington on October 23, 1786. In it, he refers to "a body of 12 or 15,000 desperate & unprincipled men" who have thrown the state of Massachusetts into disorder.
We should note, first of all, that Knox claims to be an eyewitness to these events. When writing to Washington, he states that he had traveled to Massachusetts, as far as Boston. In his correspondence, he is not relying on the reports of others, but on a subject that he has experienced first hand.
Knox begins by calling attention to the fundamental weakness of the Federal Government, as it was designed under the Articles of Confederation. This is important, because he argues that the people who have entered into rebellion have exploited the government's weakness for their own interests. Knox calls this

faction and licentiousness [and states that] the fine theoretical government of Massachusetts has given way, and [seen] its laws arrested and trampled under foot.

So, clearly he views this situation as deeply destructive for public law and order. To those who might defend the rebels by suggesting that they are reacting against unfair laws or policies, he stresses that these people in fact have either never paid taxes or have but paid very little. Rather, Knox claims a more radical motivation is at play: that they are ultimately attempting

to annihilate all debts public and private and have agrarian Laws which are easily effected by the means of unfunded paper money.

In his language, he stresses how destructive this situation has been. He refers to "a formidable rebellion against reason, the principles of all government, and the very name of liberty" as well as "the violence of lawless men."
In his words and rhetoric, he stresses the trauma this experience has caused to the politics of Massachusetts.
(A side note: this letter forms only part of a larger correspondence, and discussion on the matter continues in subsequent letters. This matter discussed is relating to Shays' Rebellion.)
https://founders.archives.gov/documents/Washington/04-04-02-0274