Basana Kimbembi, 35, allegedly stabbed Jamal Mahmoud (right), 21, in a dispute over the ownership of a parcel which had been smuggled into the north London jail the day before.
The defendant denied killing the leader of The Somalis group and said he only knifed Mr Mahmoud’s friend Mohammed Ali because he was under attack, and did not mean him serious harm.
Giving evidence at the Old Bailey, Kimbembi described how he, Robert Butler and Joshua Ratner had helped another prisoner, Cisse Ewango, take delivery of a football-sized parcel.
He said the package, which had been swung into G Wing on bed sheets, contained phones, drugs and sim cards.
Mr Mahmoud, whose group controlled contraband, had confronted them about it, saying: “You are violating. Why are you doing things without my permission? You are dealing with my operation,” the court heard.
The defendant said Mr Mahmoud had demanded the parcel but backed off when he picked up a metal chair leg. When Kimbembi told Mr Ewango about the attempted robbery, he responded by saying he would sort it out.
In the morning, two of The Somalis demanded drugs and tried to intimidate Kimbembi, who told them: “Listen, it’s not my package, innit,” the court heard.
Kimbembi told jurors he only wanted a smartphone from the package so he could post videos on his Congolese political blog.
But in the end he asked Mr Ewango for an ounce of cannabis, which he planned to split with The Somalis by way of a “peace treaty”.
On the day of the killing, Kimbembi said he had been warned to be “vigilant” about The Somalis and he felt “threatened”.
Defence barrister Michael Holland QC asked: “Given what you were being told by people about what might happen to you, why didn’t you go to prison staff?”
Kimbembi said: “If I went to the police they were going to put me behind the door. They would have thought I had snitched. What other reason have they got to put me behind the door?
“Everybody will say you are a snitch and even if they moved me to another wing I will still be targeted.”
Mr Holland said: “What’s it like to be a snitch in prison?”
Kimbembi replied: “It’s not good.”
Instead, Kimbembi told how he got hold of a metal table leg and was given half an ounce of cannabis from the package.
The defendant, previously from Barking, east London, has told the jury he stabbed Mr Ali with his own hunting knife, which was similar to one he had seen delivered to the jail a few days before.
After the killing on October 18 last year, it was found discarded on the floor of a cell on the fifth level of G Wing.
Kimbembi has convictions for 15 offences, including a cash-in-transit robbery and carrying a knife, which he said made him “feel protected”.
The defendant, who takes medication for paranoid schizophrenia, told jurors he came to Britain at the age of 11 after civil war broke out in the Congo.
His parents, who were involved in politics, were believed to have since been murdered in the troubles.
After a criminal conviction, he was refused leave to remain in Britain and found himself “stateless” with no opportunity to work or claim benefits as he appealed against the decision.
Kimbembi, Ratner, 27, and Butler, 31, deny murder as well as wounding with intent to cause grievous bodily harm.
The trial was adjourned until Monday, when Kimbembi will continue his evidence.