Using Henry's corpse as decoy, Glass ambushes Fitzgerald. He chases him into the woods and they engage in a bloody fight beside a river. Glass is about to kill Fitzgerald, but remembers Hikuc's words and pushes him downstream into the hands of the Arikara. The chief, accompanied by Powaqa, scalps and kills Fitzgerald, but spares Glass. After completing his revenge, Glass retreats into the forest and experiences another flashback of his deceased wife after the death of Fitzgerald by the Arikara.
After reading Why did the Arikara spare Glass? a similar question about The Revenant arises:
Why did the Arikara kill Fitzgerald?
I understood Arikara's pursuit as chasing people who kidnapped his daughter, he did not know either Fitzgerald or Glass so why did he kill Fitzgerald by the creek?