Salutations, Simone! I have not yet expressed my views on this dilemma faced by our brethren , the Vindicator, so I will now, tho it is of little consequence:
While it may seem somewhat cowardly and dishonourable to have attacked the perpetrator of that vile crime from the back with little warning, one has to balance that against the gravity of the crime and the anguish experienced by the defenceless victim who will presumably be scarred for the rest of her mortal life. Furthermore, Paladins are mortals, as well, albeit ones who are expected to live and be measured against very high standards, hence certain degree of reasonableness and objectivity needs to be exercised before arriving at a conclusion about the Paladin's action. In short, in light of the crime and the age of the victim involved, the Vindicator's actions were justified to some extent, though he ought to factored his DM's hint into the equation and used the blunt side of his blade or the pommel of his sword to incapacitate his target so that he can face justice. And if he is truly concerned, he can take steps to monitor the outcome of his report to the local authorities, and if it seems like they are incapable of delivering justice, then he would be justified to take matters into his own hands. At which point, he would have extended due respect and regard for the local laws and law enforcers and simultaneously delievered justice as it is due to be delievered at the end of the day! - At least thats what I would've done.