Did Fischer behave dishonourably?
To my thinking, there are three world chess champions who belong to the exclusive but highly discreditable club of champions who refused to face a legitimate contender for the title they were holding.
Staunton of Great Britain and Andersen of Germany were acknowledged as the world's best chess players at the time Morphy toured Europe in the mid-19th century, but while the sporting Andersen played and lost several games to Morphy, and greatly admired the young American's ingenuity at the board, Staunton foiled every attempt by Morphy for a match, usually on health grounds, and succeeded in waiting out the time till Morphy's return to the U.S.
Alekhine unexpectedly beat the invincible Capablanca in 1927, but it was generally felt that Capablanca had grossly underestimated the then relatively unknown Alekhine, and in his supreme confidence in his technical skills, had not prepared adequately. A return match was eagerly awaited by chess lovers, but it was never to be. Alekhine, recognizing that Capablanca was a formidable opponent and, better prepared this time, could well regain his title, used every excuse in the book to avoid another encounter with him till Capablanca's death in 1944.
Fischer too refused to play Karpov, the legitimate challenger to the title in 1975. Whatever his reasons, those of us who were actively following chess at that time could not fully accept Karpov as world champion. In 1978 and 1981, he defended his title against Victor Korchnoi(!), which did not add to his credibility, then went on to lose to Kasparov in 1985.
Fischer was only 32 when he gave up his title, and his giant shadow loomed over the world champions for quite some time. No question, his action diminished chess during this period.