On the shopkeeper thing: I think there are features of D&D that can make this a surprisingly acute source of pressure.
D&D emphasises gear as an important aspect of character build. The GP system - including but not limited to starting money - is the mechanical framework for this: GPs are limited, gear has costs, so there is a mini-game of gear optimisation.
But D&D also locates this minigame within a fictional context - the players are supposed to imagine their PCs wandering from shop to shop, pulling GPs out of their pouches and buying the stuff they need, etc.
So it's easy to see why a player might come up with the idea of exploiting the fiction to endrun around the minigame constraints and get the gear by stealing it. And from there it's a pretty short journey to killling shopkeepers.
The problem is only compounded by the fact that there are some aspects of play where trading on the fiction to endrun around the mechanics is considered skilled play (eg circumventing a low STR score and hence a poor ability to open dungeon doors by using tools to break through them or knock out their hinges or whatever; using water or a level or similar to detect sloping passages without regard to the mechnical chance a dwarf or gnome has to detect them; perhaps even using a Rock to Mud spell to bury enemies in a mud-flow without having to ablate all their hit points; etc).
I'm personally not that interested in running a game focused on killing and robbing shopkeepers, but I prefer to get to that end through system modifications rather than session zero admonitions.