How to avoid killing player characters due to a single (un)lucky roll?

Especially at low levels, Pathfinder characters can usually be killed easily by a single enemy attack, provided the dice fall in said enemy’s favor. An example:

The PC has a constituion score of 10 (modifier +0) and 8 HP

The enemy is of medium size, has a strength score of 14 (modifier +2) and is wealding a quarterstaff (obviously two-handed, as a quarterstaff is a two-handed weapon)

On a hit, the enemy deals 1d6+3 damage. On a critical hit (×2 for the quarterstaff), they can hence deal up to a maximum of 18 damage, which would kill the PC immediately with a single attack! There is no way for a healer to interfere, the character will die at once.

I believe stats as described above are neither unrealistic for a PC at level 1, nor for an enemy they might encounter. However, I recently ran a one-shot that almost resulted in TPK within 30min due to two critical hits the enemies got in early on, killing two of the PCs and thus changing action economy massively in their favor.

How can I avoid that without pulling off some Deus ex Machina stunt?

To be clear about this: I always make sure in session 0 that everyone knows PC death is possible (and so far players have always agreed to it). But even knowing that, it really puts a damper on everyone’s mood if a character the player possibly spent hours on creating dies in session 1 because of a single attack by a generic enemy.

Simply fudging the enemies rolls is not really a possibility, since I’m using Roll20 for my games at the moment and all players are able to see my rolls (including modifiers) once made. There is the possibility to hide rolls from players (which I sometimes use for opposed checks), but I would prefer not to do that for all attack/damage rolls just to be sure, because that already implies I intend to fudge a roll at some point (which I probably would in said situation, but I’d prefer players not to think about that constantly).

While this question might seem related, the problem at hand is not encounter balance (as pointed out above), but dealing with the risk of a balanced enemy killing a player due to (bad) luck.