Fudging The Rolls & Killing PCs

I've fudged plenty of dice rolls in order to balance unbalanced encounters. I run a pretty "fly by the seat of my pants" campaign when it comes to encounters. 

I judge how the roll effects the story or the encounter. I have fudged dice to keep from killing PCs, to make opposing skill checks turn out how the story needs them to, and to put a bit of extra challenge in the players' faces. I will usually roll damage in the open, and skill checks who's outcome I want to be random. I almost always do "to hit" rolls behind the screen, in case I need to fudge something.

I'd say I fudge the dice at least once every session.

When they face a fair challenge, and are not up to it. The cause could be rash action, poor planning, bad luck, etc.

I just don't want the PCs to die wantonly, pointlessly, or through poor decisions made by me, as the DM. I'd prefer they die dramatically, or at least realistically. When I killed two PCs in our current campaign, one died fair and square in battle, the other died by mouthing off to Big Baddie when he was a prisoner and had only 1 HP. Both failed death saves, as per 5e rules.

I'm still not sure how I feel about the death save mechanics. It sure adds tension when a PC is unconscious, but it also puts their lives firmly in the hands of the dice.

All my rolls happen in the open, so there's no fudging, ever.

I think my current players have had some gotcha DMs, or somehow became accustomed to an Us vs DM mentality. I was rolling in the open before I ever met them, but continue to do so because they seem paranoid, as if they don't cover all their bases every single time that I'm going to find some oversight to exploit and bring them suffering -- I would only do this occasionally! They're just now getting used to the idea they don't need to spend half an hour describing all the precautions they make before bedding down in the wilderness, or even an inn for that matter.

Accordingly, player deaths normally only happen when fate (the dice) dictate. Most of my combats would rate on the hard-deadly difficulty, but it hasn't come up in my newest campaign (level 4 now). I'm pretty solid on telegraphing danger, so the players haven't done anything outright stupid (aside from trying to sneak past a Balor at level 3, which amazingly, they did -- group stealth check, everyone rolled over 14 against passive perception 13). However, I wouldn't rescue someone who jumped off a cliff, or directly engaged the aforementioned Balor.

I have a tendency to keep combat rolls open, and more social based rolls behind the screen. Things like diplomacy or bluff should be hidden, I have also taken to rolling some rolls for players behind the screen on occasion to help keep the mystery/suspense up. I lean towards the same idea as Friartook in that if it is because of a fair fight, or the players being boneheads. Essentially the DM has the guiding hand for how deadly a campaign will be. If you put traps up that are meant to kill PCs and they kill one, well that makes sense, or an encounter they should bypass rather than face head-on and they die that makes sense to me too. If the dice swing against them it is the best way to represent the deadliness of the world they live in. If you fight a bloodthirsty scimitar wielding orc there is always a chance that you may die, and that needs to be represented somehow right?