The thing is that for an awful lot of genres, the "kill them and take their stuff" works very well. Better, in fact, than the genre appropriate actions.
This is primarily because many genres try to screw with the protagonists within the terms of the genre. And if you kill the genre and take its stuff, then the genre is left without an answer.
I don't know about that...sometimes an answer will present itself if you think about it.
If you kill & take in the West, you may become an outlaw. Or you may have just killed an outlaw...who was related to another, nastier outlaw who is "gonna come gunnin'" for the man who shot his
___________(fill in the blank).
In a traditional supers game, killers quickly get branded outlaws, and again, face being hunted by the law and other costumed vigilantes.
If you kill & take in the future, what you take may have some kind of tracking device, letting the owner (or their successors in interest) track you down, kill you and take your stuff. This is especially true if the people you're killing are police or military.
In fact, in future-tech situations, you may not even be able to get what you take to work. SG: Atlantis had its "Ancient Gene", but even in RW tech, we're talking about "smart gun" technologies that require the wielder to have a RFID chip on their person to get the thing to work. Given that said chips are smaller than the tip of an eraser, they can be (and have been) hidden in rings, badges, rank pins, name tags, belt-buckles, holsters, and other gear.