Probably the stupidest thing I’ve done in game.

[MeerkatRabbit]

That's not so bad! At least the bear didn't kill you. I lost my last two characters to bears. On my first character, I found a bear, attacked it and it killed me. Now that was pretty stupid. On the next character, a bear walked right into my just-finished, brand-new log cabin and slaughtered me in my bed.

[PALU]

I agree attacking a bear is stupid (I've done that once, very early on to see if it was a stupid idea and received the answer that yes, it is a stupid idea, so I've never done it since, although characters have been attacked by bears as I've tried to avoid a fight. Bears caught in traps are killed with thrown rocks), but getting attacked is bad luck.

Hm, come to think of it, I actually HAVE attacked a bear with my latest character: I thought closing the door and firing at it through the window would be safe, but I found out bears can open doors (not smash it: open it). Luckily, neither my character nor dog received a scratch, possibly because the arrows had injured it so it kept missing.

[Dungeon Smash]

You're not a real Kaumo until you kill a bear in melee 

[PALU]

But my character is Reemi!

[Dungeon Smash]

Carry on, then

[Míriel]

The most stupid thing I ever did was kill my dog while trying to lay a killing blow on a badger. I'm not even sure how it happened, I was returning to the village where I've left meat to smoke and in the pine mire on the way I've found a badger, I started chasing it until it was breathless and then I knocked it unconscious and followed with a killing blow, my dog was barking leashed behind me to the best of my knowledge but somehow the blow landed on her, killing her instantly. Shocked, I used the look command over the tile and saw it contained the still unconscious badger and my dead dog.
I can't stand over an unconscious animal but my dog can, or maybe just for small animals that's what I think must be the reason for this.
And this was stupid because badger spoils aren't even that valuable, fur and meat accounted, and my big (actual size description, her name was Nala) dog had helped me take down two bears unscathed and will continue to do so in the form of mittens which is a poor consolation  .

[PALU]

I've seen robbers and hirelings stand over unconscious humans, both opponents and from their own side.

If it's any consolation, it's not unheard of to keep mashing attacks at unconscious animals until there is a report there's nothing to attack, to find both a dead quarry and a dead dog there...
In your case, the badger is considered a predator, and dogs attack predators if they can (damaging their skin), so I assume the dog managed to get on top of the target while attacking, somehow. This is especially probable if your character was heavily loaded and had built up a considerable fatigue from bashing the badger, as your actions will be quite slow, and the dog may be capable of performing several actions for each of yours. However, I'm not aware of targets moving while the attack is under way (moving while performing the double action of loading an arrow and firing it doesn't count: explicitly equip the arrow first and then fire unless you're completely sure friendlies can't get in the way and the target can't get behind cover (or reach you)).

[MrMotorhead]

Sympathies for your poor dog.  I have had the same sort of thing happen to me while I was trying to finish off an unconscious bear.  I cracked my dog over the head with the blunt (thankfully) end of the spear and fractured his skull.  It was nearly a disaster because he turned on me and tried to savage me.  Luckily he calmed down after I dodged.

In the future I think I'll be tying my dogs up when I close in for a killing blow on a down opponent.

[Buua]

I drowned trying to crawl back on the ice over and over while having a punt in my inventory...

[PALU]

Sympathies for your poor dog.  I have had the same sort of thing happen to me while I was trying to finish off an unconscious bear.  I cracked my dog over the head with the blunt (thankfully) end of the spear and fractured his skull.  It was nearly a disaster because he turned on me and tried to savage me.  Luckily he calmed down after I dodged.

In the future I think I'll be tying my dogs up when I close in for a killing blow on a down opponent.

You only need to tie up your dogs with predators or when you keep them starving. In the first case they'll attack the predator (harming the skin, but possibly preserving yours), while in the second they will eat a fresh kill (again harming the skin).

I drowned trying to crawl back on the ice over and over while having a punt in my inventory...

I don't think the punt would actually help, but shedding weight from the inventory surely does. And once you manage to get up onto the ice you may be able to get into the punt to smash yourself to the shore (but crawling, abandoning the punt, might be better, as it's probably faster, and you need to get a warming fire going soon, unless your character has really warm clothing (which, again, contributes to your encumbrance, so it might help once you actually get out of the water, but it will hinder you from actually getting out...).