I agree that it's very alarming how quick Buffy is to go for the kill. But I don't think it's actually double standard-y of her. Here's what I mean:
We know that for Buffy to justify killing a vampire or demon, it must be 1) powerful, not incapacitated in any way, and 2)currently killing. (Added bonus if said creature is about to destroy the world and needs to be sacrificed in its place). These are the standards she's upheld very consistently throughout the series.
In season 4 when Anya returns to the lives of the scoobies, she's just recently killed all of them in a Wishverse. To Buffy, she's nothing more than a demon that, on an ordinary day, would need to be killed. But Buffy would never consider killing her at this point, because she is powerless. Human, for all intents and purposes. So she accepts her, (by basically ignoring her), into her circle, humoring Xander.
By season 7, they have a bit of a history, yes. But Anya has just qualified to be killed, by Buffy's standards, on points 1 and 2. So, having to make the difficult, calculated decisions as she does, she chooses to kill her. From a logical standpoint it makes a lot of sense.
It's not a double standard, because Buffy applied the exact same criteria to the vampires in her life. Angel is pretty straightforward, and Spike? As soon as he was powerless, humanlike, and not killing, Buffy couldn't kill him. Didn't qualify either point. In season 4 when she thinks for a moment that he's removed his chip, she does come after him--(also taking the leap that he will kill, which perhaps isn't fair but also isn't a logical stretch). Then, back to the state of things as they were. In 7, Spike is under the influence of the First. It isn't simply her feelings for him that keep her from making a rash decision on his part. His actions are very clearly different from Anya, who chose to become a demon again, (as a direct parallel to Spike choosing to get his soul at the very same time), and she began killing again.
So Buffy deserves credit here for her consistency, even if the calmness with which she goes about it is quite alarming.