Thoughts on: Fudging Rolls to improve your GMing?!

Essence: In my experience if rolls are fudged by GMs it happens in the favor of the players. But how about cheating to their disadvantage? Could that add to your game?

Details: Why do GMs roll attacks or damage behind a screen? I can think of only one reason: to fudge rolls.
Interestingly enough this is almost always done in the players favor. Mostly to prevent a character from dying.
As a player I hate that! If I get the impression that the GM is cheating to prevent that my character is killed I lose the interest in the encounter. And believe me, players tend to notice such things (a question about the current amount of HPs is like an announcement that the GM will be cheating anytime soon). Combat should be lethal and if my character cannot die why bother with combat at all? And since I do not like that rolls are fudged to my advantage as a player, I do not do it as a GM either.
If a combat turns out to be too hard, I either withhold enemies the characters might not have yet seen for that encounter, go along with every idea the players have to turn the combat to their advantage and if I cannot the players to rethink their tactics (e.g. by withdrawing) I cut the HP amount of the monsters as a last resort. Maybe it is not better than fudging dice but I hope it is more discreet.
In John Wick's 'Play Dirty' he speaks about cheating against the players favor in order to punish them. I do not like the notion but it got me started thinking if there are situations where it would add to the game if you cheated to players disadvantage.
What if you came up with a idea to make a combat more dramatic, create a awesome/funny scene or give a monster a unique taste but that would require a successful attack. How about a bull rush that would push a character at the edge of a cliff and while the melee continues there every blow has the risk of falling. Or what about monsters with very iconic attacks like a Medusa or a Rust Monster. Only if one of their distinct attacks hit do these monster stick out of the normal crowd of thugs. But does that justifies cheating?

As a note: Even if I have not gotten a lot else from 'Play Dirty' it at least made me think how I GM and I am more willing to give 'dirty' methods a try to improve the game.