My fighter against an assassin who we suspected killed an NPC in the first session.
We are down in the mud, grappling. I fumbled my sword and he got me good (crit), leaving me quite vulnerable. The other party members were nearby but not in range.
I pulled out a dagger and got a crit of my own, ending his life with less than 5 hp left of my own.
Later that night I realized that dagger I'd killed him with was looted from the NPC's pack. I'd gotten revenge on her behalf, using her own knife against her murderer.