I think that to describe Old Mahon as "cranky" is something of an understatement. It would be far more accurate to describe him as a mean, abusive drunk who terrorized his son Christy throughout his childhood. It may well be the case, as a previous educator has noted, that Old Mahon was himself subjected to such an abusive upbringing. But this in no way justifies his appalling treatment of Christy. Old Mahon had a choice to live his life in a certain way, and he chose to be the Father From Hell.
That's not to say that Christy would've been justified in killing Old Mahon, but at the very least the old man's abusiveness does garner Christy some sympathy. More than that, it gives the tall story he spins to the awestruck villagers a veneer of plausibility. One senses that such abusive behavior by fathers towards their unfortunate offspring is by no means unusual in this neck of the woods. That would explain why the villagers are so quick to relate to Christy's story, and to his false confession of murder.