The irony was that the Kray twins had actually paid Jack McVittie to murder a fellow criminal called Lesley Payne who they feared would inform on them.
The twins were too high profile to be linked to the murder so they contracted the killing out to McVittie. Not only did he fail to kill Payne, he refused to return the money paid for the hit. Incensed, they murdered McVittie with a room full of witnesses.
This moment of madness led to their being sentenced to life imprisonment...