A paid MI 5 informer or insolent, undercover policeman makes a...

 ...good liar but a bad witness if his background is in politics or English literature. Why? He has good articulation but bad mentoring skills. In court,he engages in obvious flights of fancy and in 'civvy life' concupiscence with his principal's opponents. Then,he often divulges confidential information. He is a friend of every Tom,Dick and Harry he meets.

 The best police witness is a scientist. In court,a doctor or nurse often diverges widely from the truth to exculpate himself from blatant malapropisms and serious crimes. Those crimes are clearly the main cause of  open defiance of civil norms in Ireland and Germany. Even occasionally,too, France these days because of the atavistic urges of the police and the media in the entertainment age.