I like the idea of Winter betraying someone but I don't think closing Atlas borders would be the cause.

It would be an interesting add to the story as well as Winter's arc.

I still think that it would be interesting if she were revealed to be another one of Salem's servants/allies. After all, after living in Atlas in the Schnee Manor and serving under Ironwood, she's probably seen the worst of humanity: on one hand, a self-serving élite of douchebags that has no idea about what common people go through and that exploits the lower classes to no end; on the other, a sly and irresponsible general who has betrayed the entire concept of independent Huntsmen, constantly threatens democracy with his two seats in the council and, despite being part of a secret group of "protectors" of mankind, has his mind set only on Atlas and nothing else. What's there not to betray?

Hmm good points but what exactly would Winter gain from all this? Power for herself?

Winter being the mole would also explain a lot of things: how could the WF steal so many Atlas prototypes? How could Cinder prepare the virus if not even Ozpin, Ironwood's close friend and boss, knew about the general bringing an entire army to Vale for the tournament?

And why Qrow doesn't like her. In all seriousness, that would explain some things if Winter was/is siding with Salem.