I'd prefer to see a number of possible results to be determined by the AI. The raiders should have at least three options:
1. Conquest. Take the settlement more or less intact (the inhabitants included) and occupy it until such time as you or some other party takes it from them.
2. Destruction. Plunder the settlement, ravage, enslave, or kill the inhabitants still inside, destroy the settlement completely, and return to "where they came from".
3. Raiding / Killing spree. The raiders kill every last man, woman, and child that they find, and strip the corpses. Perhaps they don't feel strong enough to hold what they've taken, just don't feel like hanging around, or already have a base of operations. Whatever the reason, they haul ass after damaging your settlement to some extent..
"Revenge is a dish best served cold"..
When you show up and see the settlement that you built under new mangement- or in ruins with the bodies of the NPCs that made that settlement worthwhile lying scattered around, you'll know what to do. In cases where the raiders don't stay on site, the player should be able to obtain something regarding where they went.Said information should be time sensitive. The more time that you take to gather rocket launchers, mini-nukes, and more followers, the less likely the raiders will be inclined to hang arould waiting to hammered. Throw enslaved former inhabitants into the equation, and a moral gray area of sorts arises. You either decide to go after the bad guys and erase them from the pages of history, or some survivor decides to erase a sizable chunk of your karma by spreading the word that you lacked the big ones to avenge your own friends. In any case, your best chance to avenge the dead and free any captives should be if you go with what you have and pursure them immediately.