The DM is not the player's enemy

It is easy for DMs to assume that his/her's job is to kill the PCs. Now, don't get me wrong, I think that the DM should challenge the players, but there is a difference between a difficult encounter and giving level 1 PCs an inescapable, fight to the death encounter with 5 level 9 Red Dragons. A DM should make the game balanced, but still difficult. What's the point of giving a group of newly-started PCs a task for a level 12 PC, or tasking an epic tier group to kill a giant rat?
Not randomly killing PCs, however, should NOT be equivalent to fudging if at all possible. If you set a gigantic,fearsome, immortal creature against PCs who, it turns out, get their asses handed to them, it makes no sense for it to deal massive amounts of damage in the first 3 turns, then deal 3 hp damage on its 4th, just because "It bumped its head against a tree and is now bloodied. Also, it is confused and it does less damage." This is and obviously lame excuse for fudging rolls because the PCs were having trouble killing a monster you should have not made so powerful in the first place, or even worst, you fudged it just so that "Y'all can survive." If you were really fighting an immortal cyclops, do you think it would only gently swing its spiked club just because you were dying? No! This sort of fudging only reduces the fun of D&D(not that I know anything about it; I haven't bought a single thing related to D&D but I still think it probably would). Even if you really got it wrong and really tried to make a suitable monster, give a more plausible excuse like,"Suddenly, the ground shudders. It's an earthquake! The (insert monster name here)takes (insert reasonable number here) damage." This, of course should not be abused just so that bad DMs can randomly put powerful monsters in a group of weak PCs' way.
In conclusion: make the game fair and balanced, and try not to fudge rolls to help players if you didn't make the game fair and balanced.