To add to my previous statement...
I'm not an a-hole; there have been times when I've tried to cut the PCs a break. It does not happen very often, but usually it happens when something I designed (when I played D&D*) with the intent of being easy turns out to be overly hard in practice. However, I don't like to fudge dice, so I usually try to prevent the issue ahead of time or giving the PCs an alternate option. Though, sometimes, the PCs seem to insist upon doing something no matter how obvious I make it that they shouldn't. Once we get to the actual rolling of the dice, I tend to let them fall.
I'll also say that -to some extent- it depends upon what system** I am playing. One of the reasons I like to let dice fall where they may is because I don't like to GM with the assumption that the PCs should be able to kill (hack & slash) through everything. I like the idea that PCs are a cut above the average joe, but I don't feel that means there won't be times when they run into something or someone who can match or better them. It is my opinion (and my experience) that this encourages PCs to give some thought to their decisions and consider options from the character's viewpoint rather than playing by the mantra of "well, we're PCs, so..." or "the GM wouldn't put it here if we couldn't kill it..." or "having my character swim in feces is no big deal; the GM won't let me die of disease***"
I highly agree with what has been said about a TPK not needing to be death. Not every monster is necessarily out to kill the PCs. More intelligent villains might have other plans for them; yet still, a more ego driven villain may wish to provide the cliche evil monologue about his plans before finishing them off. To take this a step further, I will even say that death need not be the end. In the most recent D&D campaign I've GMed, the PCs were dead and had to play through the Raven Queen's realm. Eventually, they earned their way back to the land of the living.
* With not intention of starting debates or arguments, it is my view that the current version of D&D has certain assumptions about what PCs should be doing, and many players (even if it's subconciously) are aware of this; playing accordingly. Likewise, I feel it -to some extent- implies that you should craft encounters which fall in line with those ideals. PCs are supposed to win and level up so they can do that again at the next level. This statement in no way means to imply that you cannot go against the grain; I've done so, but, generally speaking, this is how I see things.
** If I'm playing a game in which the system assumes that combat, social interaction, and other skills are all equally viable solutions to a problem, I don't worry so much about making sure every challenge is at the same physical level as the PCs. If the system is one which is more gritty, things such as disease and starvation don't necessarily fall into the category of 'weak deaths.' Such systems usually provide a way (whether through skills, items, or both) to avoid or recover from such things.
*** Yeah; this happened. I understand that different people find different things enjoyable. However... I dunno... you just had to be there I guess. It was just a little mentally draining having a character who insisted upon smearing feces upon himself at all times. Other players weren't happy either when -instead of helping fight the enemy during combat- the character tossed bags of feces at them (the PCs.) It got pretty out of hand; even after I attempted to sit down with the player away from the game and talk about it.