The English mastiff cross, who was severely burnt in a house fire in Cromwell four years ago, was put down on Monday night.
Ms Bell, of Alexandra, said Demon had recently been to Christchurch to begin specialist treatment to fix a wound that would not close, after four years of regular visits to vets in the South Island.
He had an operation to remove tissue damaged in the fire, but testing of it revealed Demon had two types of skin cancer, and Ms Bell decided to have him put down.
Demon's last week was a good week, filled with ''unlimited treats'', running wild, a bag of 50 tennis balls to chew through, and a family photo shoot.
Money donated through Ms Bell's Givealittle page paid for Demon's surgery and bills, and she appreciated the support, time and money people had given him.
''There are a lot of very kind people out there.''
