Once a ressurected spy even had to fight one of her former corpses turned into a soulless wight.
This statement brought to mind an adventure I was part of. I'll try to provide some DM insight into the OPs dilemma toward the end.
I had a mage/cleric that, with his entire party, was trapped in an underground labrynth. We found hundreds of local villagers trapped in cells. The party picked or broke open locks to free them but none of them moved. We spoke, argued, yelled, pleaded, but to no avail. They stood right where they were, mesmerized. We were about to rope them and start pulling them to an exit when a group of wights appeared. The began to duck into the different cells and drain the captives and turn them into wights as well. We set to fighting the wights! In short time, it became evident that the party was going to be completely overwhelmed if we didn't think of something...and fast.
My mage/cleric said a prayer, begging for Pelor's mercy, pointed a finger at the nearest cell that the wights had not gotten to yet, and cast a maximized fireball spell (yep, 3.5 rules). The party paused for a moment in complete shock. To which I spoke, very gravely, "Pelor is merciful." And he was. The DM did not penalize alignment (CG) and I cast several more spells to devestating affect. The slaughter was disgusting. Even OOC we all sort of reeled from the implications of what I had done. People literally fell out of their chairs because of my action. It was WAY out of character for me as a player. But all in all, it was really a 'last resort' choice for our party. Undeath is not life. And, I reasoned, Pelor would not have wanted any of those he cherished to suffer the taint of undeath. And so they died. (After the event, we had a bit of a philosophical discussion, got a little somber, but played on!) We defeated the wights, escaped the labrynth, and found our way back to town. After finding solace at the temple of Pelor, we retired to our beds at our previously acquired manor and were rewarded with dreams of souls dancing under the radiance of Pelor's light. It was a nice touch by the DM to keep the party from imploding under the weight of something so out of character for our group. And it allowed me, as a player, to feel vindicated for 'doing the right thing' in an impossible situation.
So, all that being said, I'll say this:
Our DM put us into an extremely difficult situation. From the player's perspective, it was downright impossible. The DM had NO idea that the solution would result in the slaughter of many villagers. But he did reason that without all the information available to us that, of course, he had in his head, we had to make split decisions. Split decisions by characters create chaos in the minds of DMs but good DMs will roll with it.
To answer the OPs question:
YOU put them in this situation so YOU should also provide a way for the character to restore some/most/all of their INT in some way or another that doesn't take the player out of the game. Or let the party mercy kill the character. Did you, as the DM, forsee this sort of thing happening? Did you believe that your characters would all walk away roughed up but intact? Do you have any qualms about killing characters outright? There's lots to consider as the DM. I've been there. But I've also let the characters make their own choices without penalty one way or another. It's bad enough that they lost a valued member of their party. So they could put the PC in a litter and haul the body around until they found some way to solve the problem, or, they could do the mercy killing (no penalty from the DM, but the PCs don't have to know that). Either way, you've got a player at the table with nothing to do. Usually, they're feeling sore, embarrassed, mad (maybe), or even sad at the loss of a character they've put time and energy and passion into. I usually reward the player with some sort of token honorific. Erden, one of our rangers, died under the withering fire of a radiant spell that a witch was casting in his general area. The party renamed the geographic area from "The Shallows" to "Erden's Rest" and it remained that way from that day forward. We noted it on the map and it stayed in our world. He felt a little better about the situation, rolled a new character, and the game went on.
Anyway...I ramble as you can probably tell already...what we have done in situations like this is had the player with the dead character start playing the monsters and/or NPCs for the DM until they were back in the game. This usually wasn't more than one full session. Eventually, they characters stumble across a poor lost soul in some cave somewhere. Perhaps the local farmer's son decides the life of adventure looks good and tags along with your party, or maybe the party stumbles into town bleeding, limping, and very nearly dead...yet here comes a cleric to the rescue! They befriend the cleric and he/she soon becomes part of the party. You know how that all goes down.
So let me sum up:
1. DM needs to own the situation
2. DM should allow for ANY resolution to the situation without penalty to the players if the DM set up an impossible situation.
3. DM should ALWAYS reward players in difficult situations. I'm a firm believer that XP isn't just from monsters. And...players play the game to have fun. It can be goofy fun or serious fun, but it's ALWAYS fun earning XP, gold, reputation, or honorifics. It costs you nothing as a DM and everyone continues to come to the table to enjoy the game and the social interaction.
