Heroin addict Ben Hope went about the murder of Aamir Siddiqi with "staggering incompetence", according to the prosecution.
Hope was already a known criminal and drug addict in Cardiff when the murder was carried out.
Jason Richards has a criminal past littered with violence which has landed him in prison.
He is a heroin addict who unapologetically made a living from peddling hard drugs on the streets of Cardiff.
The pair met in prison and their intimacy behind bars went on to become a drug-dependent friendship outside.
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Scotland Yard said the male youths will appear in custody at Westminster Magistrates' Court on Saturday.
Windy this weekend with gales across northwestern parts of the UK. Rain spreading southeast.
Greater Manchester Police said "inquiries are ongoing to fully understand the circumstances of the incident."