The Memory of a Killer
aka: The Alzheimer Affair
aka: The Alzheimer Case
(2003)

Ledda (Jan Declair) an aging hit man with Alzheimer's, agrees to one last job. When he learns that one of his targets is a 12 year old girl, he angrily refuses to complete the assignment. After hearing that the girl was murdered regardless, Ledda, using the last ounce of his skills, begins to clean house, murdering everyone that was involved in the girl's death.

It turns out the a high ranking minister, Baron Haeck (Jo De Meyere) had ordered the girl's death to cover up the sexual abuse committed against the girl by his son Jean (who Ledda kills, among others).

Ledda is eventually killed by the police (obstinately to cover up corruption of other high ranking officials who had been in the Baron's pocket). However, before he dies, he gives Det. Vincke (Koen De Bouw), an honest cop who only wanted justice for the dead girl, a clue to where damning evidence against the Baron is hidden. Vincke recovers the evidence and the film ends with him watching Baron Haeck get arrested for sex trafficking and murder.  

Thanks Tim (Spectre)!