Mrs Guarino, a mother of two and grandmother of four, was bludgeoned to death by her 24-year-old lodger Marc Riley.
He and his gay lover, David Carr-Burstow, 19, then disposed of her body by throwing it in the River Avon.
Her body was not recovered for 40 days, but within 24 hours of her being reported missing, DS Fulcher had arrested the pair and they were subsequently charged in connection with the murder.
Riley eventually pleaded guilty to murder and obstructing a coroner, while Carr-Burstow was acquitted of murder but admitted obstructing a coroner.
Mr Sebastiano said DS Fulcher's support throughout the investigation and subsequent trial had been a huge comfort and said the idea his career was now in jeopardy for doing his job was "disgusting".
Mr Sebastiano told the Daily Telegraph: "The whole investigation was incredibly traumatic and complex.
"Within a couple of days of us reporting mum missing, the police had arrested two suspects, but that was just the beginning for us.
"My mother's body was not found for another 40 days. Even then we had the trauma of the trial to cope with and what was ultimately a very unsatisfying outcome for us.
"One of the suspects pleaded guilty but the other denied murder and was eventually acquitted. Throughout the whole difficult and complex process Steve Fulcher was there supporting us and holding our hand.
"He was understanding, patient and sympathetic. As a police officer his job was to catch the people responsible but he seemed to go above and beyond the call of duty to ensure we got the support we needed.
"He really helped us through what was an immensely difficult period. The idea that his skills and experience and compassion are going to be lost simply because the rules are not fit for purpose is simply outrageous.
"I am so angry about this. No one seems to care about the rights of victims. It is all about the rights of the criminals. Now that is even taking precedence over the right of the police to do their jobs."
A close friend of DS Fulcher, who asked not to be named, also warned that the police force could ill afford to lose an officer of his talent and experience.
He said: "Steve is a husband and a father and that it always at the core of his police work. That does not mean he lets his emotions cloud his judgment, but he is always thinking about the victims and their families.
"Effectively he is being condemned for this when he should be getting praise. At a time when morale in policing is at an all-time low, this is the last thing that the service needs."
DS Fulcher was the senior investigating officer tasked with finding missing Sian O'Callaghan who disappeared after leaving a Swindon nightclub in March last year.
Taxi driver Halliwell was quickly identified as a suspect and after being placed under surveillance was arrested.
But rather than taking him to a police station and questioning him in the presence of a lawyer, DS Fulcher subjected him to an urgent interview, in the hope he may still find Miss O'Callaghan alive.
After initially refusing to co-operate Halliwell eventually relented and led the police to not only Miss O'Callaghan but also the remains of 20-year-old Becky Godden-Edwards, who he killed between 2002 and 2005.
However because of the failure the procedures as laid down in the Police and Criminal Evidence Act, Halliwell has not been charged with her murder.
Despite this her mother Karen Edwards has hailed DS Fulcher a "hero" for the way he handled the case.
