Shortbow range is extremely low. I wouldn't even bother unless they're within 10 tiles, preferably within 7-8 tiles or less. Also, shortbows are not good for combat at all. They are emergency-level survival bows. You might get lucky and kill some game now and then, but not suitable for combat. As for why your spear thrusts failed, I would say bad luck. Or, possible the nerp had really high axe skill (or both). On the other hand, I would say you survive your other combat due to good luck.
In this game, combat against a human opponent is by far the riskiest activity available. You should not view this as a standard roguelike, where combat is a major component and is meant to be balanced. In this game, It is meant to be highly unpredictable and dangerous. Even high-level characters with quality armor and weapons should approach every encounter with the utmost caution, and seek to maximize every advantage (outnumber your opponents, bring dogs, strike from surprise). Even then, you can easily be killed through bad luck. Starting characters should view combat as an absolute last resort. Going into combat when you are outnumbered is a sure way to die. Some players do make a game out of hunting nerps, it is certainly possible. But not the focus of the game.
I would also like to second this description, playing in the past as someone who experimented with stat cheats and the like, you can still be eliminated with double the maximum attribute values and double the maximum skill values if you make foolish mistakes, which may not all seem to obvious but this game's combat mechanics are scary accurate to what would happen in a real situation, using default values. Even characters with a mastered skill have a high chance of dying, the difference is they also have a higher chance of killing their enemy. My advice is to think of the game as if your character was a realistic thing, there's only so much you can do with the knowledge your character has, and it may not be enough to effectively fight an enemy with a weapon and armor. In my experience, dogs are your best friend when it comes to combat. (which obviously makes sense in a realistic situation as well).