Norman Llewellyn killed the animal with a single kick during an angry row with his fiancee, Julie Blyth, at her home Cirencester, Glos.
Despite Mr Llewellyn, 52, from Hertfordshire, admitting criminal damage he accepted a caution for causing suffering to Phoebe the dog, which was a gift to his fiancee.
He was also facing possible RSPCA prosecution but the animal charity decided not to charge him with ill treatment because the dog died instantly and did not suffer.
Beverley Carpenter, Ms Blyth's sister, criticised the decision.
"The RSPCA told us they could not do any more about it," she said.
"We think it is disgusting. My sister is devastated about it. It seems no-one wants to know - and we are upset he has got away with it.
"What sort of message does it send out to people?"
The RSPCA defended its decision.
Spokeswoman Judith Haw said: "The RSPCA is deeply troubled by the violent death of this pet dog and investigated immediately once alerted by the police.
"A forensic veterinary surgeon carried out a post-mortem which concluded that the dog died instantly and did not suffer.
"Evidence of suffering is necessary for cruelty to be proven in the courts and as an expert ruled that suffering did not occur, animal welfare offences could not be considered."
She added: "The Society fully understands and shares the public's concern and frustration but of course we can only act within the law.
"It would be an irresponsible use of charity funds to try to proceed with any case which had no evidence to support it."
A Gloucestershire police spokeswoman said they decided not to prosecute him because he showed remorse and had no previous convictions.
She added: "An independent investigation in relation to animal welfare was carried out by the RSPCA, fully supported by Gloucestershire Constabulary.
"The conclusion of this examination was that the dog died instantly and did not suffer, therefore no animal welfare offences could be considered."