It would be wrong to suggest that Snape was not a man of action, but he didn't do anything impetuously. His actions were planned, studious; time was not an issue.
He probably learned this at his parents' house under the influence of his brutal, unpredictable father. Sheer power was not the way to survive a man like Tobias Snape; you had to use cunning, guile, stealth; you had to be patient, but always alert, always on guard.
Perhaps his original attraction to Tom Riddle had to do with the allure of power, the shuffling off of the feelings of helplessness. Did he think that he, a half-blood, could gain a favored position in the New Wizarding World Order? Even after it became plain that this new order involved torturing and killing people, Snape did not act impetuously; it took a threat to the only woman he loved to make make him take the action of changing his allegiance.
Now he feels that he can only watch and do nothing. Matters have gone far beyond his control. Yet he can do something. He can protect the students to a certain degree from the Carrows, transmit the real sword eventually to Harry, and throw a red herring to Bellatrix.
And if the alternative to watchful waiting is impetuosity, he has already convinced himself that that is a losing strategy also. In the end, of course, the winning strategy was a combination of careful planning and gutsy action. But Snape did not live long enough to see that.
This story continues the author's string of well-written fiction, a good expansion on details of the canon story, an effective combination of action and analysis. I reviewed the scene of the death of Charity Burbage in Book 7; I had always assumed that Voldemort did the deed, but this author assumes it was Snape. The book does not exactly say.
We never fully appreciate what Snape did for everyone else, do we?
This was a great story! I loved how tragic it was that Snape had to watch all the time but not intervene. I also found it highly funny that Snape put the sword in Bellatrix's vault, because he had no idea that it would cause trouble later when Harry ended up at Malfoy manner with the real sword... :)