The parallels between Willow and Warren are particularly noticeable in the scene in "Villains" where Willow creates an image of Katrina to taunt Warren. What “Katrina” says to Warren, and what Willow says to Warren about Katrina. Change “Warren” to “Willow” and “killed her” to “erased her memories”, and everything "Katrina" says, word-for-word, that is said could be applied to Willow and Tara. Presumably, this is Willow's subconscious guilt speaking. This fascinating vid illustrates the parallels between Willow and Warren.
The Warren-Spike parallels start as early as “I Was Made to Love You”, with the Buffy-bot. As someone online once pointed out, Warren and Spike both give up their robots, but they don’t give up their warped ideas about romance. They are on opposite trajectories: Warren going down, and Spike going up. And they both get there through attempted rape.
In the scene where Willow taunts Warren with a vision of Katrina, you could change “Warren” to “Spike” and “killed her” to “tried to rape her” and then everything "Katrina" says, word-for-word, could be applied to Spike and Buffy.
There is one difference between Spike and Warren, which probably helps to explain why the former gets love from fandom, and the latter doesn’t (well, okay, this and James Marsters' good looks). When Katrina asks, "How could you say you loved me, and do that to me?” Warren replies, so suddenly that it startles: “Because you deserved it, bitch!” It’s an instinctive reply. It’s very revealing.
But Spike has a very different instinctive reply about what Buffy deserves in a Joss-written scene in “Hell’s Bells”. When he sees that coming seeing him with a date genuinely upsets her, he offers to leave.
"No,” Buffy says. “No, I ... you have every right to be here. I pretty much deserve—”
Spike cuts her off. “That's not true, you...” He looks at the ceiling, a frustrated by his own reluctance to hurt her (foreshadowing his frustration with his guilt in the crypt scene in "Seeing Red"). “God, this is hard.”
There is also Spike’s last words of the season: “So you'll give me what I want. Make me what I was. So Buffy can get what she deserves.” I had been spoiled for the fact that Spike gets a soul, but I also knew there was controversy in fandom about whether or not Spike chose to get the soul, or was tricked into it. It was this line that made me realize it was his choice. I could believe that Spike would have the chip taken out so that he could kill Buffy and her loved ones, but I could not believe that he would think she deserved it.
Also, despite the parallels between Spike and Warren, Spike’s initial reaction to his realization that what he had tried to do was rape is more like Jonathan and Andrew’s that Warren's--shock and denial.
Despite Faith's absence in the seasons were Warren is present and alive, there are also Warren and Faith parallels. It’s obvious that Steven DeKnight re-watched “Consequences” before writing “Dead Things”. Many others have pointed out the parallels between the argument between Buffy and Spike in the alley, and Buffy and Faith’s arguments in “Consequences”, but I don’t know that I’ve seen anyone discuss the ways in Warren's reaction to killing Katrina mirrors Faith's.
Both Faith and Warren start their downhill spiral by killing someone accidentally. How they get there is very different--Faith, despite a lot of reckless behavior, was doing her job as a slayer, ridding the world of vampires. Warren was trying to become a master criminal and force his ex-girlfriend to take him back. But their reactions to the unintentional killings are similar: Their first instincts are to take no responsibility and dump the body. They deny feeling guilty about the person they killed (a denial that is more convincing in Warren's case than in Faith's). They are quick to emphasize to the people who were by their side (and who arguably share responsibility for the death) that the guilt must be shared. Faith tells Buffy, “You were right there beside me when this whole thing went down. Anything I have to answer for, you do, too. You're a part of this, B. All the way.”
When Jonathan asks in shock what Warren has done, Warren tells him, “We did this. Me, and Andrew, and you. It's on all of us.”
Both Warren and Faith try to frame the same person, Buffy, for the crime, refusing to consider the possibility of taking responsibility themselves. “I'm not going to jail,” Warren tells Jonathan and Andrew when they suggest going to the police. When Buffy suggests that it will be worse for Faith if she doesn't come clean to Giles and Wesley about what she has done, Faith denies that anything could be worse than jail for the rest of her young life. Perhaps there is a difference here--Faith assumes she will get the harshest consequence possible; Warren doesn't want to accept any consequences at all. But for both of them, the refusal to accept consequences leads to the same result: Now there is nothing to stand between either Warren/Faith and further acts of murder and mayhem.
In some ways, Faith has more in common with Andrew, who thinks getting away with murder is "kinda cool", but hadn't seriously considered doing it before. Angel warns Faith that “to kill without remorse is to feel like a god." Killing opened up for Faith possibilities that she hadn't considered before. For a girl who had been abused, neglected, and powerless for much of her life, the ability to take a human life offered a kind of power that she could only imagine before.
For both Warren and Faith, their first genuinely evil act is attempted rape--although in Faith's case her assault of Xander happens after her accidental killing of Finch, rather than being prelude to the accidental murder as Warren's mind control of Katrina was.
Both Faith and Warren end up not-so different: A core Scoobie goes after them, looking for blood after Faith and Warren attack her lover. Willow and Buffy have different motivations--Willow is out for revenge for Tara's death, Buffy is trying to save Angel's life. Both have different results:Willow succeeds in killing Warren, who cannot match her power. Buffy and Faith are equally matched, and despite losing their battle, Faith is able to foil Buffy's plans and escape with her life, even if she's stuck in a coma. Her death is only metaphorical.
Neither Scoobie is that repentant about their actions. Buffy and Willow may have some remorse about their act of murder/attempted murder, but neither has a lot of regrets about who that act was directed towards.
“A murderous, misogynist man. I mean, do you understand what he did?” Willow asks Kennedy, in “The Killer In Me”. She says, "I killed him for a reason.”
“You had it coming,” Buffy tells Faith in “This Year’s Girl”.
But while Warren's death ends any chance he might have had at redemption, and he reappears only to tempt Andrew to the dark side and represent Willow's inner darkness, Faith gets a second chance, and is inspired by her experience being Buffy and Angel's example and support to come back from the dark side.