There's a handful of reasons:
1. Willow is a human being. It's just not slayer's jurisdiction, at least according to what Buffy says. If Willow wants to kill another human it's not something Buffy should intervene in as the slayer, she wasn't created to protect humans from other humans after all, she wasn't created to kill humans, she was created to die to protect them. Buffy can and should intervene as a friend but when she intervenes as the slayer she righteously gets her ass kicked.
2. Buffy's not capable of taking out Willow, Willow's too powerful but more importantly, Buffy just isn't capable of killing her like she wasn't capable of killing Dawn. It's just not an option. You know, I think that people don't realize just how much Buffy loves Willow and Xander.
3. Angelus and Spike are walking cadavers, monstrosities, vampires - you don't kill them, you slay them, banish the demon that reanimates a dead human body. It's not the same as taking a life of a human being, like Willow.
There is no double standard. Spike and Angel are not analogical to Willow - Dawn and Ben are.
[MENTION=2739]Buffy Summers[/MENTION]
or Vamp Willow for that matter
Because Willow told her not to. Alright, I'll try to keep it brief. The relationship between Buffy and Willow is based on two things: love and power. Buffy basically created Willow, took her under her wings, nurtured her with her love and attention but there were strings attached - Buffy was the boss of Willow. As the show progresses this dynamic is slowly turned around and in season 6 ( Which should really be named
Willow The Series or something by the way ) it is Willow who is the boss of Buffy. Buffy's life and death becomes a matter of Willow's decision ( twice! ) regardless of what Buffy wants and when somebody has that kind of power over you they pretty much own you, in a boss-like fashion. Now, Buffy during the majorly pissed off Willow arc still thinks that she is the boss of Willow and Willow proves her so, so, so very wrong. It's such wonderful humiliation, it's better than anything Spike ever did. OK, but when did the shift happen? When did Buffy started losing her power? When did it change? In "Doppelgängland" obviously.
characters on the show criticize Buffy for her unwillingness to kill Angelus, but no one criticizes her for not taking out Evil Willow
Which character should do that? Xander? Dawn? They love Willow. Anya and Spike? They're not exactly in the position to bitch about it. Giles? Nobody cares about his opinion.