I think it's a little bit of both.
In the end it's players who are essentially metagaming and have discovered that killing things and taking their stuff is applicable at all parts of the game. Wizard has a quest for you? Find out what he wants, kill him, take his stuff, then take the quest-loot for yourself. Town guards won't let you in? Kill them. Shopkeeper has an unreasonably priced magic item? Kill him.
But at the end of the day, this style of "playing" and I will only tentatively call it "playing" D&D since I dislike it so aggressively, is enabled by DMs. It is incentivised by systems and settings and DMs who only reward XP from "loot" or "combat" and no social encounters. It's supported by DMs who fail to create any sort of system to hold players accountable, be it a tough force of "Hobo Hunters" or even the most basic of reputation systems to promote society-positive behavior. It is a way of "playing" enabled by overly restrictive DMs as much as too lenient ones.
So that is to say, murderhoboing isn't entirely on the players. There are systems fundamentally embedded in the game and thus, compounded in many settings that promote an attitude of "kill it and take its stuff" over alternative solutions.