Funny, I was reading about this not long ago!

Benedict Arnold was, indeed, one of the most celebrated military generals at one point. After his participation in the Battles of Saratoga in 1777, where he brought about the surrender of British General Burgoyne and his troops, Washington gave Arnold a gift. He gave him a sword knot and epaulets, a gift from a French official who had asked Washington to give them to one of Washington's "favorite military generals". Arnold was greeted by his hometown of New Haven, CT as a hero, and he was appointed to major-general (albeit belatedly).

Because of a leg wound he received at the battles, however, he could no longer serve in the field. Still, he was appointed the military governor of Philadelphia in 1778, and lived lavishly while hosting parties for the city's elite. It was here that things started to go wrong.

In 1779, the Pennsylvania Council accused him of shady financial dealings and abusing military privilege. To be fair, he was using his access to military supplies and other things to gain an edge in the merchant trade, which others didn't quite like. Washington reprimanded him after a long and bitter court-martial that followed, and after he'd resigned his position, at which point he resolved to join the British. The opulent lifestyle he had was something that he was attacked over, though other colonists who could live that type of lifestyle were doing so (and making similar abuses in the merchant trade and speculative markets).

But back to the point; what about the Continental Congress? Well, Arnold would typically say that his sacrifice and help in the war effort should've been enough that they should forgive his occasional insubordination and unbalanced finances. He believed that the Congress was unappreciative of his successes in war, and overly critical of everything else. However, there's been some who suggest that he was simply blowing them out of proportion: it's likely that no one would've remembered those conflicts had he not committed treason. It seemed more a matter of pride that he took such affront and would loudly argue with the Continental Congress, which was sure to hurt his standing there. However, he had also amassed a fair amount of debt due to those unbalanced accounts, and Congress at the time was very strapped for money, while he was living an opulent lifestyle. It was a toxic mix.

Source:

Lori J. Ducharme and Gary Alan Fine. The Construction of Nonpersonhood and Demonization: Commemorating the Traitorous Reputation of Benedict Arnold Social Forces, Vol. 73, No. 4 (Jun., 1995), pp. 1309-1331