Raiders. Possibly the second-biggest post z-day threat after zombies. This page is dedicated to running through scenarios where raiders are involved, like what would happen if a large group of them overran a military base and now all of a sudden had heavy weapons? We'll start the discussion on that note.

One option to counter a group of hostile raiders with military tech is to catch them by surprise. It shouldn't be all that hard if you have a functioning colony/fortress/group. That is of course assuming that you have scouts out and about. Or at least are monitoring radio chatter and keeping an eye out on the major roads. The idea is to wait until they stop moving for the night (if they have captured armor), then circle them as much as you can and ambush them. If nothing goes wrong you'll catch them with their pants down (maybe literally), and then have access to whatever they stole, assuming that the government isn't tracking the stuff. If everything goes wrong, well you might make it out alive. Or you could be blown to bits. Or forced to give up your base.

If the raiders in question didn't take any heavy armor, then you could still use the night-ambush idea, but it might be more practical to ambush them on the road when their guard might be down. People have a tendancy to set watches at night, so a road-side ambush might be ideal. The only trick would be to find a suitable choke point (or lacking that at least get some cover) and to pick off the most dangerous-looking guys first.

Large groups of raiders may form a large colony, complete with zombie pits, mines, snipers, and suspended cages for prisoners or slaves. This would probably be in a smaller town, such as Hartford, Connecticut, which is the headquarters for Colt. If a colony of well-meaning survivors come upon a cache of weapons, they become a large band of careless raiders who prey upon all others.

Raiders are not all necessarily murderers, most are just survivors who can't solve their problems with the mind, so they use the gun, the knife, or other weapons to get supplies. There will be certain raiders who actually are killing any and all who cross their paths, but they will be among the ranks of the undead or killed by police-like forces.