So, here's the **THEORY:** Minerva McGonagall is an amazing actress, but she's also deeply and passionately a Death Eater—probably even one of Voldemort's most trusted and loyal followers. As such, she sometimes shows her true colors. Considering her age, she could have known Voldemort during their school days. As a spy for the Death Eaters, it is her job to suck up to Dumbledore. But—unlike Snape—she is highly unwilling to do anything to help the Order. Perhaps she's bidding her time; perhaps she's foiling them more than we know.
So, let's get down to it. Let's painstakingly go through the books and watch McGonagall very carefully.
“Minerva McGonagall is Evil: The Ultimate Essay [1]”
Having read all the books except the last one (which is just now coming out), I've never given Professor McGonagall much thought in the entire series, but this essay does lay out some good arguments for her being on the side of evil.
It's something to keep in mind …