You've been asked by the local nobility to go and slay the local tribe of savage humanoids. So of course your group of good-aligned adventurers tromps off through the wilderness to put a stop to the possible onslaught of monsters. You get there, find a group of level-appropriate orcs, armed with some level-appropriate equipment they found somewhere. They blindly charge and you slaughter the whole group, loot their dungeon, and tromp happily back to town.
Everyone is happy right?
Savage humanoids are a staple of the tabletop RPG genre. They have many shapes: Orcs, Goblins, Bugbears, Kobolds, Troglodytes, Lizardmen, whatever. The problem is that a lot of DMs treat them as above. You don't need a reason to slaughter them: They are GOBLINS. They exist, as an all-male, all-adult, all-evil collection of monsters to slaughter. Either that, or they do something blatantly evil to give motivation to a set of adventurers. They slaughter a village and leave a burning ruin. They raid caravans. Eat babies. Kick puppies. Don't rewind their rental tapes.
That's fine, I guess, but the villains and the world feel more real if the events that happen have some real motivation behind them. "Being Evil" doesn't count.
Why do they do this? Are savage humanoids just to lazy to make their own stuff? Being evil doesn't mean you have to pick on people that you know will kill you. Orcish and Goblinoid oral histories have to be FULL of tribes that got stupid and picked on human settlements, only to be obliterated by a group of three to six humanoids. Dead down to the last man, woman, and child. Tents burned, stones scattered, dungeon plundered, history destroyed. Why would any tribe be so stupid again?
Every setting that includes savages tends to say that they exist in great numbers across large areas, usually mountain ranges. Now, mountains aren't really arable, but that's not the point. When did you last see a goblin farmer? But the point I'm aiming for here is that they obviously have some way of feeding themselves, or there wouldn't be so many. They have some way of working stone, or wood, or some other material, because all those people need shelter. They have alliances, enemies, and political intrigue of some sort, even inside their own civilization. Why would such a people randomly ask for destruction by assaulting human cities? Though life might be hard, they aren't dying off.
Well, there are reasons of course. I can't just pose questions and not answer them.
1. Jealousy. This is, in a lot of ways, the underlying cause of most human conflict as well. "Yeah, we are doing OK over here, but those guys have gold streets that flow with milk and honey. I want their stuff without having to work for it!" The orc tribe living in the Darktimbre Forest look across the farms to the human city that glitters with prosperity, and they look down at their half-rotten deer meet, and they bristle. They want gold-plated buildings. They want high walls and roofs over their heads. They want farmer slaves, since to a slaving culture, it seems that's what they are. They want to lay about in the city like noblemen while others do all their work.
2. Resources. Effectively the other primary cause of human conflict. Whereas in the above, they have basically what they need, here they don't. Sure there are millions of orcs in the mountains, but that's exactly the problem. They've over reproduced. There are no more cougars to hunt, no more bones to gnaw, no more booze to funnel down their throats. Then, they see those human trade caravans roll by, guards bristling with weapons, but the carts loaded with a few tons of food and drink. The orc patrol hasn't eaten for three days, and their canteens are empty. Desperation sinks in, and they assault the humans. They eat the people, eat the stored food, guzzle the wine, and laugh as they wear new human chainmail over their full bellies. Then they realize something. They have a tribe of two hundred strong orcs, and caravans come through every week. What's to stop them doing this forever?
3. Religion. Arguably the cause of the remainder of human conflict. Orcs worship Gruumsh, Goblins traditionally Maglubiyet. Kobolds and whatever else generally have their patron deities as well. Gruumsh does not care for Corellan Larethian. How dare that elvish tree city worship such a heretical god? They must burn, and their tainted food and resources must burn as well. It's all tainted, and consequences be damned.
Those are all fairly basic, yes. But you are dealing with a pretty basic people. Orcs and goblins do not traditionally have a lot of INT or WIS. They see things plainly. "I'm hungry, and that human farmer has food." The goblin knows the farmer won't willingly share. So he kills him and takes it. "We're cold, but that human inn looks warm, and their people have clothing." The orc tribe of 20 knows they could live off of the reserves in that minor village for weeks. They are cold and starving in the northern winter, and so they attack. Ok, enough of motivation.
Savage humanoids are still humanoids. Despite 4e's monster manual barely mentioning noncombatants at all, orcs have families. They might be evil, but they are still capable of love. They marry, they have children. They care for the children because they have the same racial preservation instinct as every other animal. Attacking a group of raiders should be a hugely different experience than attacking the tribe's home. Raiders will fight until it looks like they will lose. Then they will flee, because the rewards from the raid are unlikely to come, and death is never preferable to life. Fathers protecting their wives and children will fight to their dying breath, and with a ferocity that you will never see the equal of. Attacking a group of barbarians that know you will slaughter their children should throw them all into a rage that will only end when the threat is over. They will not break, they will not surrender, and they will not back down.
For that matter, consider your good aligned party. Most parties are at least non-evil anyway. You attack a tribe. You slaughter their fighters. Let's say you stop short of killing noncombatants. Most parties will loot the bodies in front of their terrified loved ones, and then head back to town to sell the stuff they picked up. This is not good aligned behavior. Besides, many of the children will take up arms if they watch their fathers get killed. Now you have an implacable foe that is equivalent to a ten year old boy holding a sword. He can kill you if you ignore him; a sword is a weapon in any hands. But it is an inherently evil act to kill children. What do you do?
So give the next group of orcs some thought. They need a reason to do what they are doing. They need to seem more human in their dwellings. They need family. Your enemy isn't just "Orc #23", he's "Muzgob Goretooth, Orc Fighter, worshipper of Gruumsh, husband of Melgri Goretooth, and father of three".
It will make the game more interesting, I promise you.