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今までのようにただ帰って行ったんじゃないぞ。

Someone (I think it was a native Japanese speaker) translated this for me as "They didn't just go home like they always have". But I don't think this makes any sense at all. My translation is "It just had to leave like it always did", but it might not be accurate, either.

Context: a monster was attacking a group of people. They fought back, and it left.

What's a better way to translate this? Thanks in advance!

Isn’t there some previous history about this monster? To me it sounds like, „well, this time the monster didn’t just go home as it used to do“ implying that normally it doesn’t attack or leaves pretty quickly but that this time hit some reason the monster was more ferocious. Maybe they upset the monster? Or attacked it?

I believe the “native Japanese speaker” translated it correctly.
今までのようにただ帰って行った”んじゃない”ぞ。
There is definitely that negative, saying it DIDN’T go back in the way it always/usually did.
To me it sounds like they want to say that this time the monster didn’t go back in the way it usually did, meaning, one of two opposite scenarios:
(1) the group of people fought back good that the monster really felt that people resisted well (so that it felt defeated, maybe thinking it is not worth coming again),
or
(2) maybe (depending the context) the monster took away a good loot (which it used not to)? That would also fit the sentence “it didn’t go back simply as it did before.”

And Admin, please move this thread to “Language” section.

The monster had appeared and attacked the group before. It killed some of the people in the group before leaving, and this happened a few times. This time, the monster was met with somewhat fiercer resistance and simply left without killing anyone.

I still can't understand what this this dialogue is supposed to be saying, even though at first glance, it's a very simple sentence. XD

Then I would take it as:
- It didn’t go back like it always did – this time it left knowing we are here to fight back.

今までのようにただ帰って行ったんじゃないぞ。

"They didn't just go home like they always have".

That would be correct in a different context, but not in the context that you explain, because it's not a "they".

"It just had to leave like it always did"

I'm afraid that is definitely incorrect.

The speaker is implying that the monster had always been leaving in a certain way, but that this time, something is different. じゃないぞ is a denial.

The speaker does not exactly say what the "difference" is. So, it's natural to assume that the readers/listeners, along with the speaker and his allies, are supposed to anticipate what's going to happen next. In other words, it seems to me that this is a scene where you're supposed to think, "What? What's so different this time? What's going to happen?"

If you as a translator have no way to find out what exactly is going to happen or what is so different from the monster's previous behaviors, all you could do is to translate it so that it would work in various situations. For example, "It's going to be different. It didn't just leave."