It's been awhile — and I don't have any of the books available now so as to quote them on this — but during my long tenure as an RPGer, my understanding was that experience points were awarded for "defeating" non-player-characters and monsters.

Keyword in that sentence is "defeating." Not "killing."

In my group (where I was GM more often than not), XP were given for doing ANYTHING that somehow "bested" one's foes. That included sneaking past them, diverting them, stealing from them, driving them off, talking them out of trouble, snaring them in a trap, swindling them, whatever. As a group we came up with some really creative strategies and antics to avoid pitched battles (so many that I hardly know where to begin recounting them).

We also received XP for any successful feat which was in any way unusual. This included eluding traps, solving puzzles or riddles, haggling for a good bargain, etc. Over the years we took pride in NOT solving problems with combat. A "really good night" for us, was one in which a d20 "to hit" was never rolled.

I suppose if one's creativity is limited or one is extremely literal in one's reading, one might actually believe that D&D was intended to be a slaughter-fest ... but we never thought so, and I know no long-time players who ever did. Had it been the case, I guarantee I'd never have been interested at all. This objection, then, is pretty vacuous.