Benedict Arnold's betrayal of the Continental Army is quite clear ["America has brand new Benedict Arnold"]. He was snubbed out as leader of the troops for the attack on Ticonderoga by Ethan Allen and the Green Mountain Boys and in the end got no credit at all for the Continental victory, which he should've gotten.
And there was the Battle of Saratoga. Horatio Gates refused to go at the British, so Arnold led the attack, suffering a bad wound to his leg and becoming a true hero – but Gates the coward got the credit.
The cherry on the sundae was when he received a letter of rebuke from Gen. Washington (which the latter was forced to write).
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So, we DO know why Arnold betrayed America. We don't condone it, but we do understand why. As for Injustice Roberts, that is a mystery!