Is this supposed to happen?

I found a random encounter where some imperial soldiers were escorting a prisoner. I opted to free the prisoner and that's when the soldiers attacked. The prisoner joined in, punching the soldiers with his bare hands. I killed them all and then the prisoner suddenly turned on me and started punching me. He then spoke, and this was the really strange bit. The subtitles read:

"Imperial Soldier: I'll get you for that!"

So what, is he an undercover imperial soldier involved in some entrapment scheme or something? Or is this a bug?

Sorry if this has been asked about before, I expected it would have happened to someone else before. But a thorough forum and google search turned up absolutely nothing even remotely related to my problem, making me wonder if it was intentional and I'm just too thick to see it.

The caption " imperial..." I have never experienced before, but the prisoner turning against me has. I'm pretty sure it was the result of an accidental blow he took while killing off the soldiers.

I suppose it's possible, though I did take great care not to hit him and he was on the other side of the road from me most of the time. I guess the tip of my sword might have skiffed him.

I just wonder if this event will have any effect on the rest of the game? Wouold there normally have been a quest initiated or a dialogue started from freeing the prisoner? I don't know because I don't liike to look at spoilers, but in this case I might go look purely to see if my game is now a wee bit broken as a result of this.

EDIT: Turns out it's not a big deal.

But I am still curious about him sounding like a soldier. That was a bit strange but kind of funny too.

If the Imperials were leading him then he was a Stormcloak prisoner. You must have whacked him without knowing and that is why he turned against you and called you an Imperial. Just kill him too and you will see a note that all witnesses have now been killed so you will be safe to play without repercussions.