Santre Sanchez Gayle was only 15 when he shot Gulistan Subasi, 28. Judge Stephen Kramer said his "youth and immaturity" were partly to blame for his being willing to commit murder for 200 pounds ($320), but he looked like a pro on the videotape captured by a security camera, The Guardian reported.
"Anyone seeing the short but telling CCTV clip of the shooting cannot but be struck by the chillingly deliberate and cold-blooded way in which you went about your business," Kramer said.
Gayle, 16, and Izak Billy, 22, who recruited him, both received life sentences. Kramer said Gayle must serve at least 20 years and Billy 22.
They were convicted Monday. Serdar Ozbek, Subasi's partner and father of her 7-year-old son, was acquitted of arranging the killing.
Two of Gayle's relatives are already imprisoned for murder, and police suggested he was trying to be as tough as them, The Sun reported. Prosecutors said Gayle spent the money he got on clothes.
During the trial, prosecutors said Ozbe sought a contract killer and the word was passed until it reached a member of the Kensal Green Boys, Billy, who passed it on to Gayle.
"When we saw the CCTV we were convinced we were looking for a professional," said Detective Chief Inspector Jackie Sebire. "There was no hesitation and he was totally confident."