Havent tried this yet but here is some info I found if anyone else is having the same problem:
I got the same bug as you, and just worked it out!
So apparently, there's a bug...
Like me, after speaking to the slaughtered hunters, you left that "cave" and saw a quest marker to some random place on the map. There, you found sinding in werewolf form walking. You have a short convo with him, and decide to help him kill the hunters. Then, he just stays there, and keeps saying "Didn't expect to see you again.".
At this point, i thought the hunters would just show up or soemthing, and then we'd kill them together. Nope, he just stays there...
So what I did, is i went back to the cave where the dead hunters were. He actually appeared right behind me as soon as i entered the cave and was a following member of my party. We roamed through the cave (up these sort-of-hidden steps) and eventually killed a few waves of attackers. After killing the last hunter together, the quest is complete.
I think the quest was supposed to make Sinding disappear after talking to him on the world map, and then the quest marker should have moved back to the cave. Unfortunately, it didn't, and you need to figure out to go back to that cave yourself.
If you decide NOT to help sinding, he will start running away from you on the world map, and you have to chase him down and kill him.