Is it possible that some random monster killed him?

Usually, monsters will never attack the dying sage no matter what, even if you command them to do it. Maybe one of the prerelease changes unintentionally modified this behavior.

It is possible for a trap to be triggered and kill him. Perhaps something disturbed the chaos servants and they triggered it.

I saw khelly die from a fireball explosion triggered by a chaos servant once.

I cried a little.

That's unfortunate. I never had any traps generated in Kelly's room. Maybe they could be made to never spawn there?

Fixed. There will be no more traps on this level.

Just had a sad situation where I reached the room with Khelavaster and chaos servants. Right next to the sage, there was a fire vortex.
Some of the chaos servants were neutral while the vortex was hostile.
I was hoping to lure it out of the room but it exploded almost right away and killed poor Khelly before I had the chance to do anything.
I had an amulet of life saving and really needed the scrolls but sadly, that option was taken away from me as:
Is it possible to make him immune to vortices? I'm not sure if making him immune to all elements is the right path but at least somehow protect him from those vortices because this is a ridiculous situation.

Here's an alternate idea that could also be extended beyond just a special case for Khelavaster...

How about a new room effect, that causes all explosions to be prevented (say, with the message "This room seems to be pressurised"). This would include Ball spells, explosions from alchemy, fireball traps, and Vortices. They could be quite rare, but with the room containing Khelavaster being a guaranteed such room. I'm imagining that all of the explosion effects are handled by either one single piece of code (as in, a function that gets called with location, size, and type of explosion), or by a set of such pieces of code for each type of explosion - in which case, a simple check for this room type would be performed within such code.

sometimes you just get shit luck and your game is screwed - I don't think we need to prevent all the possible ways of screwing your game with Khelly. It sucks but it's life.

This is very unfortunate... but it doesn't really happen often, right? I'd also ask how you'd justify him having elemental immunities, but this is already the "dying" sage that can deflect thrown moloch armors with a weak hand motion, so that's secondary.

The gameplay-focused side of the issue is that, if he's made immune to the elements, then any PC that can cast a ball spell can just walk into the room and start balling everything, without concern for Khelavaster's wellbeing.

Yes, I think the tweak of removing traps in that room is fine, but making him elemental immune would be overdoing it.

I'v eno also immunized Khelavaster to vortices to prevent that kind of frustration. But I'm really not going to immunize him to everything.

The room is filled with chaos servants... they are weak and do not require the attention of ball spells, even a level sub-20 wizard wielding a random eternium weapon can clear the room using entrance as a choke point, without casting a single spell.
Maybe the room should just be treated differently such as ID staircase rooms that have no items generated inside.
Khelavaster's room could have a similar mechanism where no other monsters but the chaos servants can be generated.
If a vortex wandered in and killed him then I can understand that shit happens but this vortex was generated right next to him and exploded as soon as it saw me.

I'm highly looking forward to see what message is printed when a vortex blows up in his face :)

This.

The RNG giveth and taketh away. Sometimes you save the puppy and sometimes it dies on the same turn as when you enter PC6.

Losing the puppy does not have far fetched consequences for the rest of the game.
Losing Khelavaster before talking to him means he won't be able to instruct the PC in what has to be done regarding the orbs, if the PC doesn't already know.
As far as I know, Khelly is the only one to explain the situation with orbs, bar a few poems by the minstrel.
I think this is a good idea to make Khelly more resistant to accidental deaths.
Khelly doesn't give any great rewards after talking too him and letting him die, only when he's saved does the player receive the real rewards.
You need an aols already to do that which is why I think that "shit happens" rule should not apply here.
He is clearly a lawful being and the chaos servants around him are chaotic yet they don't attack/kill him because he's meant to be found alive by the player and spoken to.
Making him immune to vortices is the right choice and has no drawbacks, either story-wise or gameplay-wise.