Multiple killer Daniel Gonzalez said voices in his head told him "to keep knifing them and knifing them" as he stabbed four people to death.
Marie Harding was the first to die during the three-day killing spree
His murder trial at the Old Bailey heard evidence on Tuesday about how Mr Gonzalez "jabbed" Kevin Molloy several times in the stomach in north London.
He is also accused of murdering Jean and Derek Robinson at their home in London, and Marie Harding in Sussex.
The defence argues he is only guilty of diminished responsibility manslaughter.
Lawyers for Mr Gonzalez, 25, from Woking in Surrey, claim he was suffering from auditory hallucinations at the time of the four killings in September 2004.
During police interviews, he said the voices in his head were "like Chinese water torture".
Speaking about the moment he attacked Mr Molloy in a north London road, Mr Gonzalez said: "My head was going all mad and that so as soon as I see him - I was actually quite flipping pleased that he popped by - I just run up to him, popped him, stabbed him up.
"I had to flipping murder someone then, I had to get someone done.
"I wouldn't be able to have been satisfying myself otherwise, wouldn't be able to think properly otherwise.
"All these thoughts were coming into my head, so I just went 'duh-duh-duh-duh' and just jabbed him about three, four, five, six, seven times in the stomach."
The defendant told police officers he also slit Mr Molloy's throat.
Mr Gonzalez has admitted the attempted murder of two other people who survived being attacked by him in London and Portsmouth.
He denies four counts of murder, but the prosecution alleges he wanted to become a famous serial killer by launching a campaign of murder from the south coast to London.
The trial continues.