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.

In The Playboy of the Western World by J. M. Synge, the first indication we get of the relationship between Christy and Old Mahon is that Christy thinks he killed his father and is proud of the fact rather than remorseful. This shows that there has been a history of a bad relationship based on bullying and resentment. From the attitudes of the villagers, this appears to be socially acceptable (albeit not when it escalates to present murder). Thus I think we are getting a portrait of how abusive behaviour is passed down across generations, i.e. that Old Mahon was probably abused by his father and then replicates that model with his son. There is also a sense of an omnipresent culture of heavy drinking in the play, and thus some of Old Mahon's behaviour can be accounted for by alcohol abuse.