Quest cannot be completed

The quest “Les intrus” (in French ; second quest given by “Sebastian le condottière” in Sassau’s monastery) cannot be completed because I killed the Chief of the camp and loot him and que Quest statuts still says “Kill the chief of the camp”.
How can there be that sort of bug after so long ?

Do you have the “Lost and found” quest?

Yes, but as I didn’t kown, went back to the camp and I looted the chief (I missed him because the fight occurs in the bushes and there was 7 or 8 bandits).
The loot didn’t change the status of the quest. It’s a pity that if I forgot something I cannot go back and continue and I must do this (unrealistic) quest.
I took an older savegame to complete it.

You have to wait up to 14 ingame days, then the camp leader should be respawned
Like any NPC u can kill