First of all, it depends on the setting and how the DM has defined and expressed the various alignments to the players. Because the truth is, what the alignments expect, by the books, isn't clear on these things. Which means that different people define "good" and "evil" and "lawful" and "chaotic" and "neutral" in different ways.
I hold that it's not fair of a DM to rule against a player's actions on alignment questions if the DM hasn't made it clear beforehand what the alignment expects.
Consider this: If, as the game indicates, an orc is always evil, and thus is an inherently evil creature in the same manner as a demon, then how is it evil to slay a captured orc? Is it evil to slay a captured demon? Is it evil to cast cure disease, and thus slay smallpox? What's the difference? That the orc appears to have intelligence? So does the demon. (And so does an AI. Is it therefore wrong to turn off a computer?)
If the orc is essentially programmed to do evil, no matter what, then how is it wrong to end that evil outright?
So, then, the question isn't really about the rightness or wrongness of killing the orc in and of itself, it's about the nature and circumstances of the killing, isn't it? If I can kill an evil orc in a fight and have that be okay, and kill an orc from ambush with a bow and arrow and have that be okay, or slit the throat of a sleeping orc sentry and have that be okay, how is it wrong to do the same to an orc who's kneeling in front of me, given that it is always and irredeemably evil? What has made the difference? Clearly it's not helplessness— the unaware orc in the ambush is effectively helpless against my attack, as is the sleeping sentry. So then the objection must be made in that the orc surrendered and I accepted the surrender.
And that's the key. The PC has accepted the orc surrender; thus, the burden of the situation of the PC is whether or not he will honor his own decision and his own word. That the target is an orc or is evil is now irrelevant. What is relevant is the integrity of the PC. A lawful good PC must uphold his own integrity.
Now, interestingly, surrender, in and of itself, is no guarantee of survival. In fact, surrendering may merely be the delay before an execution. Is that not the case with a murderer surrendering to the town guard? The murderer knows he will be tried, found guilty, and executed under the laws of the town. The lawful good PC who acts as part of the town guard knows this as well. And yet, surrender is called for and surrender is given and accepted. Why?
And then, what happens if, after every surrender, the party simply executes the surrendered foes? Will not word get around that the party takes no prisoners? And thus, will not foes then cease to attempt surrender at all, but rather fight to the death or flee? That, too, is realistic.
My solution for all of this is to make it quite clear to the player of a paladin or similar PC exactly what the Paladinic Code for his character's order is. Thus, the player (and the PC) have guidelines for how to handle such situations. Then the breaking of alignment is never a "gotcha" moment (as too many DMs of poor quality treat it). Instead, it is a deliberate choice by the player, aware of the consequences and ready to bear them.
So now we are back to the surrender situation.
Part of the problem here is that too many DMs treat their monsters as stereotypical "tough guy" villains who all, to a critter, are out for revenge against any PC who defeats them, and thus always come back from a surrender ready to fight.
That's crap DMing.
Instead, the DM should consider that monsters have a sense of self-preservation (else they'd never surrender in the first place), and know when they've been thoroughly beaten— even the fanatical ones. Sure, no one can truly accept an orc's word on anything, and the orc may be evil, but if the orc just saw his buddies put through a Cuisinart of a paladin's whirling blade, flambéed by a wizard's magic, and finely chopped by the dwarf's axe, when he says he'll leave and never come back, he probably intends to do just that. Survival is his instinct, especially if he knows there are no other orc buddies to rally for any purpose. And the DM should allow for that to be what will happen.
The monsters aren't hard case gangsta's from a cliché tv show. They're not stupid.
So not all surrendered monsters are gonna come back, and even lousy rotten liars will keep some sort of promise if they believe it's what will save their own skins.
And let's face it— how often do the orcs ever actually return? How often do the PCs even know that a given orc has returned?
So the whole "moral dilemma" thing is, in the end, just cheap, dumb "gotcha" theatrics from a mediocre DM on a power trip.
Far better to have clear understanding of what alignments and various classes expect, so that instead of "gotcha" we get "story."