Why is it that I get down to something like 30/40 HP, then a monster hits me, and without getting a severed head or anything, I die? HPs aren't worth crap if I may get killed at any moment.
It probably gave a crit to your head or groin when said bodypart was already damaged. Every body part has its own HP, and if your head, groin, or chest go to 0 HP, you die. You can't see the HP of your parts exactly, but you can get an idea of how damaged they are by looking at the picture of your character's body in the upper-left corner (it's *not* just there to give you a quick view of equipment or to look pretty). Light red is bruised, red is seriously wounded, and gray is 'close to death'/almost out of HP. Endurance (which affects HP) is worth having, because it makes all bodyparts more resistant to damage, and that extra HP is applied to all bodyparts.
1. How do I even avoid dying in an efficient way?
One of the ways to handle this a bit more in your favor is to look at enemies before fighting them; that way you can tell whether you are fighting a kobold with a -2 balsa spear or a kobold with an octiron quarterstaff (protip: the one with the octiron qstaff will kill you in melee unless you are *very* lucky, throw daggers instead, or just run the hell away). Other things to keep in mind are not to get burdened or stressed (you will suffer a whopping 25%/50% penalty to melee, which will make you miss a lot and die), to carefully choose armor with weight limits in mind (more protection is great, but lighter is better), to avoid exploring new places after fights that leave one or more of your parts in the red (you may rest, or if you want to build Agi wander about somewhere you've already been), to avoid eating spoiled food (confusion can be deadly if you walk into the wrong fights), school food is nice for Endurance training but you better have spare food on hand (protip: cans of banana flesh are great, as they do not spoil), and you can store healing fluid in empty cans to make it last longer. I may have missed a few things but I'm sure someone will be along shortly to make them known.
2. And how do I avoid getting beheaded? Had a character killing the werewolf with the slave equipped with an iron 2-handed scimitar among other things, then a wolf comes, attacks me and I'm beheaded. Okay.
You wear a helmet. You *need* to cover *at least* your head, your torso, and your groin; if those drop to 0 or below, you are done (your arms and legs are important too, but you can heal those back; your head, not so much), and a lucky crit or powerful blow is all it will take to get to that point if you are not/under protected in any of those places (ghosts are infamous for naughty groin touching deaths). Now, if you do that and you still get beheaded on a regular basis, you obviously have gone past the point where that level of AV will protect you, and need to get a better one (not that this can always be helped due to capricious RNG happenings). Be aware that this is not a complete guarantee of safety; crits to your head may still result in nasty effects like confusion or paralysis. This game is designed around the idea of a game where you're pretty close to never safe, but there are ways to be safer, which I've just told you.