Cyrinishad
Explorer
Murderhobos, as you should all know, are adventurers who wander about, using their supernatural class powers and magic to take what they want from powerless commoners while ignoring the pitifully weak town guards.
How do you guys deal with them? There is obviously the possibility of making every town guard a 500 HP beast, or making sure every commoner is actually a shapeshifted ancient silver dragon, but honestly that would quickly get stale. After all, they hardly need to be saved by some lvl 5 adventurers if they can lay low whole kingdoms on a whim. The other solution is that there is an adventuring party every other block, each ready to spring into action when they see injustice befall their fellow humanoid, or to make some officials of justice very, very powerful and always close at hand. But what of a low fantasy world? What of a place where the group is close to the only adventuring party to walk the land, and no one has ever even seen a goblin or an orc? How could one deal with murderhobos in a world were, in a civilized context, they at the absolute top of the food chain in terms of individual power?
Such a great topic! Murderhobos have always had a special place in Cyric's heart... It amuses him to watch their pride and ego deceive them into thinking they are nigh invincible gods, as does the senseless slaughter they unleash upon the world... but his true pleasure is seeing them ground to dust when they suddenly realize they are but mere mortals, and that their fate is the same as all mortals...DEATH!

There are whispers of characters who are virtuous, who help the weak instead of trodding on them and leaving them in the mud while stealing what little belongs to them, but I do not believe in these stories; I have only seen adventurers who pillage, deceive and extort, without a shred of remorse, who will kill the NPC giving them a quest and take the reward he offers from his still-warm body, then go on the quest regardless because "Hey, we need XP! And loot!", or wait for another NPC to expose important plot points before looking at each other, shrugging and making an attack roll.
Anyway, how do you deal with murderhobos in your game, especially if it is low fantasy?
The behaviors being exhibited are combination of meta-game thinking (We need to kill to get XP!) and Chaotic Evil alignment (compulsive Murdering and Looting)... I am going to presume that the Players aren't just passive-aggressively sabotaging the game... That they are instead playing in good faith, and genuinely want to explore a campaign where they are Chaotic Evil Marauders...
There are a few things I have found to be very important to remember when preparing to DM this kind of game:
1. If the players want to be the bad guys, just let them be bad guys... Forcing them down the path of "Good" by inflating random NPCs to 500 HP, or having elite NPCs lurking off-camera to pounce on them if they're not "Good" is just adversarial DMing... There are lots of interesting stories to tell about Evil characters, and I have found frequently that players sometimes just need to get the "Evil" out of their system before they want to explore a "Good" character.
2. If you don't want your game derailed, make sure any Storyline or Quest that you are hoping the PCs will follow to its conclusion, are not dependent upon the PCs interacting in a non-violent way with regular NPCs... The players are Chaotic Evil Marauders, don't put them in situations with folks that are weak or vulnerable unless you are expecting the PCs to kill them and take their stuff.
3. Never underestimate the power of VENGEANCE! Murder has this funny way of creating more enemies than it eliminates... If we're using 5e rules, the Murderhobos are in way more trouble than they realize when it comes to this stuff, because MOBS are powerful things that can take down even the mightiest Chaotic Evil Marauders, and dish out some "Frontier Justice"...
4. Never underestimate the power of JUSTICE! The Long Arm of the Law probably reaches a whole lot farther, and is a whole lot stronger than Chaotic Evil Marauders realize... The funny thing about Paladins, or Police, or a Posse, is that these folks realize that they are mortal, that their job is very dangerous, and that they are going to be confronting the worst of the worst... So, they should be sure that when confronting a violent gang of Chaotic Evil Marauders, they should be using maximum and overwhelming force, and be using the best people, with best equipment, and best training...
5. Never underestimate the power of the BBEG! There is nothing more annoying to a BBEG, than a bunch of troublesome PCs mucking up his carefully laid Evil Plans... especially a bunch of amateur Chaotic Evil Marauders that didn't get the BBEGs permission to Loot & Pillage (it's called organized crime for a reason)... and even if he would have given his permission, I suspect that the Chaotic Evil Marauders wouldn't give the BBEG his fair share (All the Loot!)... So, any BBEG worth his salt would quickly do the math, and maniacally laugh at the equation he has just written to gain ultimate power: BBEG + JUSTICE + VENGEANCE = DEAD PCs
If the PCs want to play a Chaotic Evil campaign, it would be prudent of them to remember the old adage: If you live by the Sword, you die by the Sword... And when they eventually meet their inevitable end, hopefully they don't cry more than any of the NPCs did when they mercilessly cut them down.