Paul Smith was previously charged with the murder of the missing Glasgow businesswoman, along with Colin Coats, and Philip Wade, both 42, and David Parker, 38.
Smith and Parker had their pleas of not guilty accepted last week when they admitted assaulting her and attempting to defeat the ends of justice.
Smith, 47, has given evidence against his two former co-accused.
Wade's defence lawyer, Gary Allan, QC, said Smith had "everything to gain" by choosing to plead guilty to a lesser charge and giving evidence for the Crown against the two men.
He referred to a statement given to police by Sean Alexander, a friend of Parker and Smith, which said: "David came down right away (to my house) and him and Paul went into my spare room.
"I heard Paul saying they are going down for murder and they would have to fire Philip Wade and his pal in."
Mr Allan said: "That has been your plan right from the outset, to look after yourself, and that would mean you would fire somebody in for murder, that somebody being Philip Wade and Colin Coats?"
Smith said: "You look after yourself in these circumstances. I know I haven't murdered anybody and if there's a lesser plea obviously you would take it – anybody would."
The trial continues.