Shadow of the Vaal Bug

I was joining random groups and completing some quests as we came across them. I chose to kill Kraityn and Alira, but save Oak. I left my group since I didn't want my choice affecting them. However after I chose to save Oak, my group killed him and I am missing the amulet to complete the next quest. Is there any way to fix this? I tried going back and killing the bandits by myself but they did not drop anything.

Well, this post can be ignored. I just joined the party of a group that had not killed him yet and picked up the item before he was killed.