What exactly did Snape do in OOTP?

[niekycrins]

No: HPFGUIDX 129250

We know that Snape became spy for DD before LV fell. DD even gives
evidence in Snape's trial. I assume therefore that LV knows that Snape
was a spy for DD, and that he refers to Snape as "the one who has left
him forever, and who has to die". So, if LV knows, how could Snape
spy? Also, since LV calls his DE by their names, they know of each other.
So what did Snape do? Could he be spying on LM, maybe LM doesn't know
of  Snape being a spy? Or maybe Snape is double-crossing, but I doubt
whether DD would fall for that

Nieky