Yes, it was Tom's opinion, but his was an informed opinion, since the Corleones did lots of business with McCluskey. But I think the larger issue in this excellent thread is: Is there honor among thieves? Puzo describes the "peculiar" police "code of honor": cops will take bribes to protect so-called "victimless crimes," but "draw the line" at "blood money," like drugs and murder. By that standard of "honor," McCluskey went way over the line. In bodyguarding Sollozzo, he was helping him to establish a major US drugs ring. And by pulling the bodyguards from Vito's hospital, he was actively and knowingly setting up a helpless man to be murdered. He would have been an acccesory to murder--a capital offense in New York at the time.
Vito was scarcely blameless, too. True, he opposed drugs, but mainly because he thought drugs would menace the police and political protection he needed for his regular businesses. And they were hardly "victimless crimes." The big bucks in the illegal gambling business come less from the odds, more from loansharking--a business of broken kneecaps or worse. And every dollar Vito got from the unions was stolen from some working stiff's union dues, which were supposed to buy a better way of life for the working man.