Niccolo Mandell was the head of security and intelligence on Vandor Prime.
He got the Sixth Doctor to steal the last surviving Tzun data core, ostensibly for the Veltrochni. In fact, he wanted it for himself so he could use the knowledge on how to build Tzun Stormblades to become rich. He offered the assassins Sha'ol and Karthakh a bonus if they would delay their contract to kill the Doctor until after the Time Lord had acquired the data core, and they agreed.
Mandell killed the Veltrochni who wanted the core, and was going to blame the Doctor and the criminals working with him for the murders. When he pursued them to Veltroch, his police officer wife Kala was with them. The shock of seeing her made him accidentally reveal his true plans, giving her no option but to arrest her own husband. (PROSE: Mission: Impractical)