Odoacer was one of the more interesting figures of the last days of the Western Roman Empire - the fellow who actually forced the last “official” Emperor out of office. Romulus Augustus was deposed, but so far as we know, Odoacer didn’t have him killed. That was a fairly decent thing to do - Romulus Augustus was a fairly weak figure, and not much of a threat to Odoacer in himself. But - and I may be going out on a limb here - I’d be willing to guess that Odoacer was not a particularly nice man. Folks in antiquity who seized power by force of arms generally weren’t. (For that matter, nor are the folks who try that today). So why spare Romulus Augustus?
My guess? The Roman Empire of that period actually had four emperors, if memory serves - junior and senior emperors for the Eastern and Western portions. Sparing Augustus was another of a set of measures Odoacer took to pay nominal deference to the surviving Western emperor, as well as the folks in the East. There’s an argument to be made, after all, that the transition from Empire to Odoacer’s rule really wasn’t much of a change at all.
But hell, I don’t know much about this period. What say the teeming masses? Feel free to turn this into a general discussion of the last days of the Western Roman Empire - political systems, culture, and so on. Let’s learn things!
Also – without researching whether there’s any evidence for Odoacer’s motives – it is entirely plausible that having an Emperor, even a juvenile deposed one, “in hand”, so to speak, was an actual and/or potential advantage in any dealings with rivals. (Cf. jockeying for possession of the person of Henry VI during the Wars of the Roses, because having the King on your side (even if he was insane and not actually validating your acts) was a potent sign of legitimacy of authority.
It might be, in part, because there was a another western emperor, Julius Nepos, who had more power and had even some official recognition.
If Romulus Augustus was killed then Odacer is just some barbarian who killed one emperor and now has to make deals with the others. But alive, Romulus Augustus could have made a useful tool if Odoacer needed him. If Julius Nepos or Zeno had started to look like a threat, Odoacer would have another emperor in his pocket to put out front as a symbol.
Odoacer then hedged his bets by both swearing fealty to Julius Nepos as emperor and asking Zeno to rule Italy in Zeno’s name, while recognizing Zeno as the true and sole emperor.
And while Odoacer wasn’t necessarily a nice guy, by all accounts, he wasn’t a total bastard. it’s not outside the realm of possibility that he didn’t want to kill a 13 year old boy if he didn’t have to.
That was my point. Odoacer was de facto ruler of Italy but the fiction was that Julius Nepos or Zeno was the real ruler and Odoacer was just a subordinate. That kept the locals relatively peaceful because they could tell themselves they were still part of the Empire under an Emperor, however distant he might be.
But if Julius Nepos or Zeno had tried to exert their theoretical authority into actual practice and treated Odoacer like a genuine subordinate, he had Romulus Augustus as a back-up. He could put him out front and declare that Romulus Augustus was now the Roman Emperor in whose name Odoacer was ruling Italy.
The thing about hostages is that you can always kill them later, if you need to.