A man was shot five times in the head at close range in an "execution" plotted by a group who had failed to convert him to Islam, a jury has heard.
Adrian Marriot's murder was a "swift and brutal response" to him falling out with the group, the Old Bailey heard.
Marcus Archer, Aaron Irving-Simpson and Marlon Stubbs, all 24, deny conspiracy to murder the man from Brixton, London.
Mr Stubbs and Mr Archer had said they had a "missionary zeal" for converting people to Islam, the prosecutor said.
Christopher Kinch QC told the jury Mr Marriot, of Swinford Gardens on the Angel Town Estate in Brixton, had told his brother David he had been threatened at gunpoint by Mr Stubbs and two other men and £500 demanded from him.
Mr Marriot and another man had then "accosted" Mr Archer at Loughborough Junction train station, Mr Kinch said.
Mr Archer "plainly felt himself threatened and at risk of something very bad happening to him", he added.
"He made contact with the other defendants and we say that you will be able to conclude that the execution of Adrian Marriot was being planned from that moment on.
It was likely other people were also involved in the "conspiracy, that is an agreement, to shoot Adrian Marriot dead", Mr Kinch said.
"He was shot five times in the head at close range.
"It was, the Crown suggest, quite clearly an execution."
Mr Kinch told the jury that after the station incident, Mr Stubbs rang Mr Marriot's sister Tara and said: "Your brother is a little tadpole. He just messed with a big shark, a whale."
Mr Marriot had known Mr Archer for several years before he and Mr Stubbs began visiting the house he shared with Tara, David and their mother, Ruth, Mr Kinch added.
"They said they were interested in getting converts to Islam and among their targets for conversion were Adrian Marriot himself, his sister Tara and her friend Jade Okai.
"Whether what they said was actually true, that they had a missionary zeal to convert people, or whether that was more of a front for getting people under their circle of influence, is a question you may have to consider as you hear the evidence."
"There may have been some element of pressure brought to bear by the two men but in due course both Tara Marriot and her friend Jade Okai agreed to take the Shahadah, a declaration of faith."
The two women had then been given hijabs, or headscarves, Mr Kinch told the jury.
And Mr Stubbs and Mr Irving-Simpson had brought round DVDs, Islamic books and copies of the Koran.
The case continues.