Police have labelled the murder of Joe Reilly a "cold-blooded summary execution" - and believe that three children may have witnessed the killers fleeing the scene.
r Reilly (43) was killed in his home at Glenwood Court, West Belfast, on Thursday evening.
Detectives believe that two men, one armed with a handgun, entered the property and forced two people in the house to lie on the floor in the kitchen.
They kept Mr Reilly in the living room, before shooting him a number of times in the chest.
The attackers then escaped down the street.
They are described as approximately six feet tall, wearing dark coats and dark trousers.
One was said to be wearing rimmed glasses and had a Celtic scarf on.
Detective Chief Inspector Peter Montgomery, who is leading the investigation, said: "It really is difficult to know how to describe what happened last night, the adjectives brutal, senseless and barbaric are inadequate."
DCI Montgomery said he understood three children were seen in the area around 8pm on Thursday night, and he needed to find out what they had seen.
He also appealed for anyone who was in Glenwood Court, Glenwood Crescent or the Laurelbank area at around 8pm on Thursday to contact police.
Officers have not yet established a motive, although dissident republicans are suspected.
The murder comes less then a week after 56-year-old Peter Lagan was shot six times in the legs in neighbouring Glenwood Drive. Republican paramilitary group Oglaigh na hEireann claimed responsibility for the attack on Mr Lagan.
DCI Montgomery said he was aware of the incident and that it would form part of the enquiry.
Chief Inspector Norman Haslett said: "There is no place for such an act in our society and certainly not in the community of west Belfast, which has been moving forward so positively in recent months and years.
"Those people in west Belfast who did what they did are just determined to drag us back into the past."
He appealed for witnesses, adding: "There may be people out there who have never contacted police before in their lives.
"I'd appeal for those folk to think about Joe Reilly, think about his family and think about what his family are now going through."
First Minister Arlene Foster and Deputy First Minister Martin McGuinness led the condemnation.
"This savage killing is senseless and has achieved nothing. The perpetrators care only for themselves," they said.
UUP MLA Doug Beattie expressed outrage and called for "the removal of the cloak of 'paramilitary grouping'."