Hector Findlay, from Lanarkshire, found that the animal, Ben, had made a mess in his newly decorated living room.
He then took the dog outside and beat the dog with a hammer and dumped his body in a wheelie bin. His family, who called the police, found the animal alive but his injuries were so bad he had to be put down.
A neighbour explained that June, Ms Findlay's wife, adored the animal and must be devastated at what happened, adding that he must have "lost the plot".
Margaret Mitchell, local Tory MSP, said: "Ordinary people will find it hard to believe that someone could do this to a defenceless animal.
"I would hope his sentence will reflect the horror the public will feel about this incident."
A spokesman for the Westie Rescue Scheme added that the charity's members hope a jail sentence will be considered.
Mr Findlay will be sentenced next month (November).