Why did the Arikara kill Fitzgerald?

Question

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?

Answer 1

It is reasonable to speculate that by the end of the film, the Arikara are aware of Glass's involvement in the rescue of Powaqa, who is clearly visible in the last scene along with Elk Dog. This may go some way to explaining as to why they not only spared Glass, but on clearly seeing that he is now locked in mortal combat with Fitzgerald they also gave him the benefit of his judgement of Fitz as a bad man.

When Glass finally submits his actual revenge back into 'the hands of the creator' and floats Fitzgerald downstream to the Arikara they can clearly see that this is not only the enemy of the man who saved Powaqua but that Fitzgerald himself appears to be a historical bane of the indigenous peoples, due to the horrific scaring on his head from the prior attempt they made to scalp him.

It appears that the Arikara killed Fitzgerald for this combination of reasons while sparing Glass for other notions, explained in the answer you provided the link for (Why did the Arikara spare Glass?).

Answer 2

The Arikara chief riding on the horse probably remembered Fitzgerald from the beginning of the film. In the battle scene between the Mountain Men and the Indians, the Arikara chief is shot off of his horse, and as soon as he hits the ground, Fitzgerald runs up to him, and kicks him in the face. The Arikara chief was merely getting revenge. Plus, Glass rescued the Chief's daughter from kidnap, and he let him live in return.