just to state my credentials. I am not a bear expert. But I did grow up in the country and see bears crossing our fields and we almost hit a bear on the way home from prom. Mostly, that means I know more than any city kid who watched the same Discovery channel show as I did, but less than a park ranger. My friend does have several bearskins adorning his walls.
Most the stories about bear encounters I've heard are:
- male bear finds hiker and mauls him because that's what they do
- hiker comes across mama bear and her cubs and mauls him
- person(s) sees bear over there a ways, and bear moves off.
Not too many encounters where a single bear encounters and attacks a group of people. A pre-enraged bear would probably stay away, because it is outnumbered and probably not threatened.
Once combat starts, it seems there has been a variety of outcomes in the real world:
- bear stops when target is dead-like
- bear moves on to next target until all threats are eliminated
- bear totally finishes off the current target before taking other action
As such, a DM could be justified with any interpretation, and it sounds rolling for it was a fair enough way to establish a random yet reasonable effect.
Remember, taking a PC down to 0 is as good as dead to a bear, who doesn't know about binding wounds or healing spells. His last attack effectively WAS the killing assault.
Another factor to keep in mind is how to simulate animal combat reasoning. Animals do NOT generally attack superior opponents. Even bears. They would posture first, to try to look bigger to scare away the threat. Even a mama bear with cubs present.
After posturing, which is their primitive threat assessment, if the enemy hasn't retreated (bigger bear, stupid human), then it goes in for the attack IF it thinks the enemy is stupid. If the animal is outnumbered or outsized (bigger bear, party of humans), then the animal will probably retreat.
Basically, animal fights happen because one side is too stupid to have measured up his oponnent and thinks he can win.
A single human in the real world is at a disadvantage because they are smaller, they can't win the posturing test. And usuaully they are weaker, so they seldom win the fight, hence it reaching the news as a bear mauling attack.
A D&D human has a lot more on its side, that a bear may not realize evens the playing field (15th level fighter vs. bear = dead bear).
I would posit that an animal has 2 combat modes. Basic combat mode where once it sees it can't win, it WILL retreat. Enraged combat mode is that the animal has committed to the fight, perhaps because it is hurt badly, desperate or trapped. In this case, the animal is less likely to retreat and is the more likely time for this to be a fight to the death.
It would probably be up to the GM to determine if the animal goes enraged, but it's this enraged mode where the fight should get lethal. Before that, the moment you see that the bear can't win (outnumbered, doing good damage and hitting well enough), the bear should be willing to run anway.