Throughout The Half-Blood Prince it seems accepted wisdom that Stan Shunpike is not a genuine Death Eater at all. Indeed, the suggestion that he might be a Death Eater is treated as a laughable prospect. It's assumed that he is merely being held for political reasons to make it look like the Ministry of Magic is making progress in arresting suspects.
"They’re not still holding Stan Shunpike, are they?" asked Harry.
"I’m afraid so," said Mr Weasley. "I know Dumbledore’s tried appealing directly to Scrimgeour about Stan...I mean, anybody who has actually interviewed him agrees that he’s about as much a Death Eater as this satsuma...but the top levels want to look as though they’re making some progress, and “three arrests” sounds better than “three mistaken arrests and releases”...but again, this is all top secret..."
(Half-Blood Prince, Chapter 16, A Very Frosty Christmas)
I was rather surprised, therefore, when Shunpike appears in the Death Eaters' ranks in The Deathly Hallows. He escaped from Azkaban as part of a mass breakout over the summer. If he were not a real Death Eater then I would've expected him to do on the run, to get as far away as possible from both Voldemort and the Ministry. His presence there might suggest that he was actually a true believer all along.
However, Harry vehemently believed that Shunpike was under the Imperius curse and was therefore unaccountable for his actions in the battle.
“I saw Stan Shunpike...You know, the bloke who was the conductor on the Knight Bus? And I tried to Disarm him instead of - well, he doesn’t know what he’s doing, does he? He must be Imperiused!”
(Deathly Hallows, Chapter 5, Fallen Warrior).
Is it likely that he was under the Imperius curse in that battle? Or was Stan Shunpike a for-real Death Eater the whole time?
Note that this question differs from the previous one, which asks about Shunpike's motivations in the previous book.