If you're rolling behind a screen, then it is easy to fudge a couple rolls in your players' direction. If you're rolling in the open, however, then that isn't going to fly.
Having been on both sides of a TPK before, I'll just say this. Don't be afraid of dealing one out to your group. Does this mean you should set up deathtraps everywhere ("Rocks fall, everyone dies.")? Of course not. But if they're going into combat, much less a secure facility or even a Zero Zone, then a TPK has to be on the table. If your group gets too cocky, too ambitious, too sloppy, or too stupid, whatever the case may be, then don't feel bad about knocking them down a couple pegs. If they wind up in an ambush because they didn't check their corners or silence the alarms, then that's too bad. If they decided to brute force a bug nest, then it sucks to be them. If they pissed off Lofwyr in person, then smack them with your dice bag as you TPK them.
If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.
There are roads which must not be followed, armies which must be not attacked, towns which must be besieged, positions which must not be contested, commands of the sovereign which must not be obeyed.
Most TPKs, in my experience, follow not because of the dice gods' fickle favor, but because players forget these two lines from The Art of War.
If the team knows the enemy, their weapons, tactics, and position, and also honestly considers their own capabilities, then a TPK becomes less likely. If they don't know the enemy's ability, but know their own, then even if overmatched, there is the potential to escape. If they don't know the enemy's ability, and don't know (or overestimate) their own, then a TPK is quite likely.
And if I have to explain the second one, then you ought to turn in your DM screen. Sometimes the only way to survive combat is to avoid it. Players who believe that, because they are PCs, they can charge the group of twenty men with assault rifles and two HMGs alone and come out fine on the other side literally NEED a TPK to give them a good dose of reality.
When done right, a TPK reinforces the game world for your players. Sure, you'll all be starting with new characters next week, but the lessons learned will stick with the group. When done maliciously, like when you purposefully spawn multiple monster attacks and disasters when playing SimCity just to see the flames (you know you've done it), then it will spark bitterness and may lead to the collapse of the group entirely. So the concerns over a TPK aren't unjustified. If it was Daimou Murphy that got them TPKd, then maybe throw them a bone to help them out (this leads into the ever popular 'wake up in prison' storyline). But as I said, if the players, either through arrogance, ignorance, or stupidity treat the situation as if they have plot armor and can't possibly be in over their heads, then a wake up call is in order.
