My players are... a little destructive. They have burned a town to the ground (slaughtering everyone in it) and just left a second town (in the same kingdom) with several buildings being lit on fire. Fictionally, and mechanically, how should I handle this? The initial town they destroyed was fictionally set up as the sole source of food and water for the entire area (they are in a desert basin). How would I mechanically show one town moving in to take over the production of food? Or the swift decline of civilization in the area because all the people are starving? Or something. I know this is a bit of a vague question, but any advice (or stories from personal experience) would be greatly appreciated!
Change the tags
When the world changes, the mechanical notes we keep to represent the world perforce also change. Dungeon World's mechanics and sheets are prescriptive and descriptive, so when something mechanically changes on the sheet it (prescriptively) changes the world, but also when the world changes, the sheet must be changed to (descriptively) match the real situation.
So when a village is razed and it was the breadbasket of the region, the other Steadings with Need (Food) will be affected, and other Steadings without that tag will soon have Need (Food) added as their supplies are stretched to cover neighbouring Steadings. (Or perhaps a Steading refuses to share, and now they have other problems, like Enmity or an unmet Need they're being denied in a trade war.) Not immediately, but unless something happens to intervene, Steadings that relied on that food should change in all kinds of ways, economically, socially, politically, and that can be tracked with changing tags.
Show the players the effects they've had
When you make a GM move, you now have a new source of inspiration: the consequences of their deeds, and the ripple effects they've had on the region. Maybe an unwelcome truth is that villages they approach all have their doors and windows shuttered and barred before they even ride into town, because their reputation precedes them and that's the best defense the villagers can muster. Maybe they miss on a Forage roll while making camp and you tell them that the forest is strangely depleted of game, despite recent signs of life. Maybe you show signs of an approaching threat with soldiers camped on the side of the road as they pass (the approaching threat being the outbreak of war between desperate Steadings). Perhaps you give an opportunity as they meet a merchant stuck outside a City's walls, and she explains to the Rogue how she's been banned due to a trade war but just needs to get a message inside to her contact, and it's worth coin. Perhaps a miss on a Supply move in the next town means that actually the goods are available and they were in the middle of the purchase, but holy moly, there is a troop of armed bounty-hunters bearing down on the group right now in the middle of the market square, what do you do?
Whatever you do, this is a rich vein to mine for content for your present and future GM moves. Keep bringing the effects of their actions into the spotlight, both good and bad — bad because thoughtless destruction obviously can have bad consequences for the perpetrators. But good effects too, because not everything is black and white — where there is destruction, there is room for something new to grow. Player really appreciate when their actions have an impact on the world, and showing them the results of their actions — expected and not, good and bad — is immensely rewarding for a group.
(Why both good and bad? Showing them a mix of good and bad effects keeps you from projecting a sense of judgment, and makes your Dungeon World more believable. And if they're destructive because they want to feel like they have an effect on the world, showing them that they have, without making the effects feel like “punishment”, can even prevent future senseless destruction by satisfying that urge.)
There would be a MASSIVE shift between local caravans, they stop shipping adventure gear and start shipping food and water into the country. If your part want's gear they need to stop disrupting commerce within the region. Also the cost of Inns and stuff like that, food and drink would sky rocket. This would happen within a few weeks of the destruction and last at least a month maybe more.
During this time the local powers would establish an influx of farmers and other frontiers people to reclaim the old town fix it and start the food production again. Also a group of investigators would no doubt be dispatched to discover the cause and find those responsible. Questions would be asked, people investigated and perhaps your party is one of a number of potential suspects.
Perhaps there are surrounding regions that would like to see this area left as a wasteland. Maybe these foreign powers approach your party and seek to fund their destruction, it might be funny to watch them try to destabilize a whole nation in a guerilla warfare against the food supplies.