I’m just trying to imagine the scene here... just one scenario that may or may not have anything to do with reality; I’m NOT a combat vet... So... The battle’s over. You’ve got dead and injured lying all around, friendlies and enemy alike mixed together. You want to secure the battlefield as quickly as possible to prevent someone jumping up and chopping your head off, or worse, your commanding officer’s head off. Removing everybody’s armor is time-consuming. So... just go through and quickly shackle all the enemy, dead or alive. Deal with them later. Only in this guy’s case, later he was dead.
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.
No, if he was wounded, you just finish him off. No Geneva (Lat. Genava) Convention back then.
Here's another possibility - what if he was a prisoner of the Romans, rather than the Germans? Perhaps an insubordinate or panic-stricken officer; the fighting occurred over a period of days, and discipline may have begun to fail at some point...
They were a rough bunch - probably just slit their throats and moved on.