As Sir Richard Buxton observed, in The Private Prosecutor as a Minister of Justice,  Crim LR 427, at p.427:
“A private prosecutor will almost by definition have a personal interest in the outcome of a case.”
The Courts have also acknowledged that many private prosecutions will be brought with mixed motives, most obviously where the prosecutor claims to be the victim of the alleged misconduct. Such a situation may call for heightened scrutiny of the process, to ensure that the case is conducted fairly, but will not of itself be a bar to the proceedings.
In R. v Bow Street MSM, ex p South Coast Shipping Company Limited  QB 645, the Divisional Court observed that,
“…if there is evidence that the defendant has been guilty of an offence then a desire to see him prosecuted and, if found guilty, punished is not an improper motive…even if [P’s] motives were mixed the court should be slow to halt a prosecution unless the conduct of the prosecution is truly oppressive”
In Serif Systems Ltd, Re  EWHC Admin 369, Auld L.J. identified a useful touchstone for consideration of this issue:
“One who uses a legal process, whether criminal or civil, against another primarily to accomplish a purpose for which it is not designed, is subject to liability to the other for harm caused by the abuse of process.’’