The worst I have ever actually played (and later had to apologize profusely for) was Tomb of Horrors. We picked it out because it was rumored to be "the hardest module ever" or something...
First of all, IMO, for the suggest character levels, I6: Ravenloft is a much harder module than S1. Strahd is a proactive villain, and Acererak not only sits back and waits, but 'plays fair' by providing honest clues, rewarding heroics, and correctly labeling the stuff in his tomb. (This is so obvious, that RttToH feels the need to provide an in game reason why he does this.) I6: Ravenloft is an almost gauranteed TPK with a good DM that plays the module straight and doesn't put on the kid gloves. With Tomb of Horrors I would expect most experienced players to get through with 50% or fewer fatalities, though granted I wouldn't expect any of them to have actually 'won' the module, just survived it.
...and we wanted a challenge. But this module wasn't challenging; it was boobytrapped for the sole purpose of destroying PCs.
No, it is challenging AND boobytrapped to destroy PCs. Your players weren't up to the challenge. That's ok.
It is challenging. No one faults your (or your players) lack of skill for not winning 'ToH'. We've been there. It severely tests even the best and most experienced of us. However you died, at least some of us made the same mistakes. If it was easy, then it wouldn't deserve its reputation.
A sphere of annihilation in a statue's mouth? Give me a break.
That really ought to be in a spoiler tag, but yeah, that's completely fair. If you crawl into a devil's mouth, then you ought to expect to be destroyed. And Acererak plays fair. It's not random or arbitrary. Random and arbitary would be if some devil's mouths were good and some were bad, or if the path to destruction was covered by an illusion that made it appear safe and inviting. That's one of several reasons why most attempts to emulate ToH fall flat. If you crawl into a pure black hole without testing to see if it is safe with a 10' pole, you get what you deserve.
Look, I sympathize with the fact that you (or rather your players) died that way. I knew a really good experienced group of players who the first time through the ToH lost the whole party to the SoA trap (party loyalty and cohesion convinced them that they had to 'go in after' the first character through so they could save him from whatever fate befell him). Having learned that Acererak the Eternal doesn't play around, they made it through his tomb the second time with no deaths.
The proper response to losing a character in ToH is to laugh about it, because it happens to just about everyone. It is challenging.
After the fourth character died MOST unfairly, the players began threatening me with violence.
Now, I'll say this. I'd never run long term characters that players were attached to on ToH. Unlike what players are used to, ToH is not stacked in their favor. And unlike most modules, ToH has very little mercy on mistakes. Your hit points won't protect you very much from errors. And saving throws can't be relied on to let you luck through the module with good dice rolls. Likewise, I'd never run long term characters on Ravenloft until they were a level or two above the suggested levels.