In the middle ages only higher ranking captives, for whom ransom was expected, were taken alive. The rest were dispatched along with enemy wounded. Dagger through the eye socket or throat did the trick. Coup de Grace or Cut of Grace. At Teutoburg it was even worse for the officers taken alive. They were sacrificed to the Germanic gods in less than quick or pleasant ways. The owner of this armor may have been left in it on an altar as something special for the festivities.
I see your point, nyuk, nyuk.
No, seriously, very plausible, thank you.