The part of HSDS in which four of the Mo Dong Heroes so easily came to believe that Cheung Mo Gei murdered Mo Dong Hero # 7 Mok Sing Guk just doesn't work for me. To me, it feels like Jin Yong made the characters behave in an out-of-character manner to force this plot point to work, but it isn't very convincing.
First, all that the four Mo Dong Heroes saw was Cheung Mo Gei coming out of an abandoned house with Mok Sing Guk's corpse on his back. One doesn't need to be an expert in forensics or jurisprudence to understand that merely being in the same room with a corpse or even carrying the corpse in one's arms means that a person is guilty of murder. The Mo Dong Heroes didn't see Cheung Mo Gei killing Mok Sing Guk. How did they come to the conclusion that Cheung Mo Gei *must* have murdered Mok Sing Guk just because Cheung happened to be carrying Mok's corpse?
Moreover, the Mo Dong Heroes knew Cheung Mo Gei's character: they had known him since he was a child to be a kind, compassionate, and moral person who would not even needlessly harm a stranger, let alone one of his beloved martial uncles. Additionally, Cheung Mo Gei had only recently performed many heroic services on the Mo Dong Sect's behalf, including saving the sect from Chiu Mun's flunkies, healing the crippled Yu Doi Nam and Yan Lei Ting, and rescuing the other heroes from Mongol captivity in Dadu (Beijing). Based on their knowledge of his character and his record of valiant acts, the Mo Dong Heroes really should have had more faith in Cheung Mo Gei's character than their rash judgment of him demonstrated.
It would be more believable, perhaps, if the Mo Dong Heroes had been portrayed as injudicious hotheads who made a habit of rushing to bad judgments, but that had never been the case. Overall, they had always been circumspect, wise, and judicious. Their rush to judgment on Cheung Mo Gei was extremely out-of-character for them and seemed very forced; it felt like a moment of PIS (Plot-Induced Stupidity) that Jin Yong could only make work by having the Mo Dong Heroes behave out of character.
The only thing that kind of, sort of makes it work at all is that the Mo Dong Heroes saw Chiu Mun in Cheung Mo Gei's company when CMG carried Mok Sing Guk's corpse out of the house. The Mo Dong Heroes only knew Chiu Mun as the Mongolian warlord's daughter who had captured and imprisoned them with the other orthodox sect members, and had tortured the wulin stalwarts (though notably, not any of the Mo Dong Sect members). Perhaps this contributed to their rush to judgment against Cheung Mo Gei, but even factoring this in...it's a very weak point upon which to construe that Cheung Mo Gei was a murderer when weighed against his good character and proven valor.
Overall, this subplot has never worked for me because it requires too much out-of-character behavior by the Mo Dong Heroes. Based on how they are portrayed for most of the rest of the story, the Mo Dong Heroes are supposed to be better than that.