Raymond Allen, 41, is on trial charged with murder over the fatal stabbing of David McArthur, 48, at a caravan park at Sanctuary Point, in southern NSW, in July 2021.
Allen admits stabbing the veteran but claims he acted in self-defence.
On Friday, Allen told the NSW Supreme Court in Wollongong he initially planned with another man to rob Mr McArthur three days before the veteran's death.
Allen could not remember why the robbery was not carried out but conceded he went to Mr McArthur's home on July 25 with the same man.
That man, the court heard, was tasked with disabling the cabin's lights, while Allen went to the residence with a knife to rob Mr McArthur of "drugs or money".
Allen said when he reached the door Mr McArthur grabbed him by the throat, cutting off his breathing, before saying, "I'm gonna kill you c***."
The alleged murderer said at that point his "reflexes just went like that" and he did not realise he had stabbed the veteran with a large kitchen knife.
"I remember just putting the knife straight," Allen told the court.
"Everything just happened so bloody quick."
Allen said the first time he saw blood was when Mr McArthur put his hand on his chest.
After the incident, which felt like it took three seconds, the accused said he "took off" from the scene.
"I wasn't really thinking," he said.
Allen denied there was planning involved in the proposed robbery of Mr McArthur, whom he described as a "real big guy" and "crazy".
Paramedics later pronounced Mr McArthur dead at the scene.
The trial continues before Justice Desmond Fagan.
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