Before Benedict Arnold turned traitor he was the most revered and patriotic military officer.
People change. Even your closest family can become traitors.
Sad but true
Arnold's leg was shattered by a musket ball in battle, and he refused amputation. Amputation would have been the better choice, because he must have endured years of extreme pain. (By the descriptions, he developed chronic osteomyelitis, and effective antibiotic treatment was more than 150 years in the future.) The pain and treatment he received might very well have affected his judgment -- but of course we can never know.
Monument to Benedict Arnold at Saratoga.