So let's analyze this for a second, if you'll indulge me.
There seems to be two separate issues that you're discussing. Feel free to correct me if I'm wrong, here. Also, since we're lacking some context on specific examples, please don't think I'm trying to mischaracterize you, here. I'm just trying to think out loud, too.
Issue one is one of tactics. It sounds like you're not happy with the manner of play that your group is employing, because it's too 'meta-gamey'. That is to say, they're using tactics that maximize their abilities, but doing so in a more tactical, less-stylistic fashion. That is to say, they enter the monster's lair, consider their options and throw down in a purely 'get them before they get us' fashion, with no consideration other than victory. I suspect, from your comments, that you'd prefer that this play differently, and a tad less strategically (i.e with more of a storied feel).
For example: your players are confronted by Baron Plotter and his minions. "
Surrender the Foozle, or suffer the consequences!" says he. The players react switfly, smiting the Baron's minions and gutting the poor Baron in an equally swift and unpleasant fashion. While they could have merely incapcitated their foes and rendered them prisoners, the thought either never occured to them, or they rejected it due to the ineffeciency and increased difficulty of 'bringing 'em back alive'. Perhaps they believed that the only way to earn x.p. (or the
most x.p.) was to slay their enemies. This sits poorly with you.
The second issue is they are employing tactics and a style that isn't meshing with the campaign style you had envisioned. I suspect, from your hints, that this is more of a swashbuckling, light-hearted style that you'd hoped for, but the PCs are hard-bitten 'take no prisoners' types. In the previous example, you expected gentlemanly or knightly behavior, not mercenary slaughter. In short, the PCs are treating storied NPCs and monsters at the bottom of the pit in the same fashion, which is entirely NOT what you wanted to have happen.
The question is, what are you shooting for? Which GENRE's conventions are you adhering to? Consider that D&D is informed by the likes of Conan , Elric and Fahfrd and the Grey Mouser as much as Tolkien or Vance. I think it would be valuable to the discussion to know what genre you're moving towards, to understand how you feel the PCs are deviating from it.
I think what may be a disconnect is that you're looking at the players as being bound to a code that they don't accept. They may just be seeing the equation from a different angle, and attacking the problem differently. In my game, there's a variation of this theory concerning the 'beer keg'. It works like this:
In an SCA event, a bunch of people were broken up into teams for a wargame of sorts. Several groups were contesting for a house that was secured, and the goal was to capture a beer keg (or keg of ale, whichever) and return to it a home base. It would prove a difficult task to get the keg home amidst the other teams who were also after it. Several teams considered different strategies to get the keg, sneak it out and sneak it back to the home base. One team stopped and said: "
Hey! If we KILL all the other teams, we can just walk in and just take the keg home with no worries." Which is exactly what they did. While the other teams were busy trying to figure out strategies to get the keg, THEY figured out a strategy that didn't involve the keg at all, and they won. Your players may be doing something similar.