For those who say the scenario is unfair:
There is a recurring theme, I see. So I want to ask about it.
Is it the results of the scenario that makes it unfair, or the set up of the scenario that makes it unfair?
It's really the combination of the two. No clues + instant nigh-unavoidable death = not fair. This is why I drew the comparison to "you walk out the left door and you catch a deadly disease." There are no clues, and it's nigh-unavoidable death. There is no in-character reason to suspect that left door, and there is not significantly more in-character reason to suspect that lever (one assumes this lever didn't just stand out by being a lever in a room, that levers exist in this party's experience that did not kill you). If it had no clue and did something less deadly and more challenging, it would be more fair. If it had big clues to it's deadly consequences, it would be more fair.
Had the scenario said the monk rolled a natural 20 and survived unharmed, would the scenario be called fair?
Just because people get lucky doesn't mean that requiring them to get the best possible "trump card" result is fair. You can win a hand of poker with a pair of fives, but that doesn't make a pair of fives a good hand.
How about if the monk rolled a natural 10 and survived unharmed, would the scenario be called fair?
Significantly more fair, because that would represent a danger overcome for the character.
Or is it the mere presence of the trap in the form of a lever that makes it unfair?
ThirdWizard and Bagpuss have both given examples of trapped levers that I'd find perfectly fair.
Would some clues have made it fair? Say, some piles of dust around the lever? How about a sign on the door saying, "Do not enter. Deadly trap within." [I'm not joking about this last thing, either. I could see the dungeon inhabitants putting such a sign on the door after a comrade or six got dusted.]
This would make it immensely more logical, and definately more fair, because it would give them a chance to avoid it. If they push the button that says DO NOT PUSH, feel free to zap 'em. This is more similar to my style of DMing where I don't balk at big explosions, but I do paint the barrels bright red and show some exploding before I jam one in a hole with a PC.
Obvious clues and warnings, but the trap has a high save DC (and/or does deadly damage).
...or...
No clues or warnings at all, but the trap has a low save DC (and/or does low damage).
Would you say the above two options are fair?
I would say they are fair enough, or at least significantly more fair. It doesn't seem as arbitrary if it makes sense. I go for the first result myself, more often than not, but the second result is an option that is still more realistic (though, IMHO, it lacks some teeth).
That still doesn't get into the rogue not finding the trap (a search DC of how high?), or the CR of the trap being taken into account with XP awards and treasure tables, or why the trap is instant-death instead of something a bit more less binary, but I think we can agree that those are secondary to the concerns about the fairness of the trap itself.