Maybe you are right, but she should have put more time into it, not just a couple of seconds. They were still five against one and Greyback was the most dangerous of all existent werewolves.
And I doubt Bellatrix was Voldemort's first Lieutenant: even if she was strong and fanatically devoted to Voldemort, she seemed to lack any organizational talent and usually didn’t use her brains, behaving like a Viking berserker who had just eaten some fly agarics. There were other characters much more cunning than her, like Lucius Malfoy or Barty Crouch jr. ( though this one, of course, couldn’t help Voldemort any more by then). And Bellatrix, as I think, could only be used as “cannon fodder”. I even wonder how she, with her manners, managed to stay alive till the end of the last book.
There is another thing I was wondering about: where was Rodolphus Lestrange during the 7-th book events? Voldemort earlier described him as one of his most important followers ( together with his wife and Barty Crouch). So why didn’t he do anything in the books ( he was only mentioned 2 or 3 times)?