xp for gold means you get less for killing a monster than you do for robbing it, and encourages a more careful exploratory approach. Modern D&D is much more of a tactical combat game than the original, where encounter design is "balanced" based on the assumption that players will be able to kill what they run into.

Was there a different xp mechanic between AD&D's gp = xp and 3+'s only killing is xp? Because I don't see how the switch to that could possibly reduce the incentive to murder.