Darla *used to be* a temptation towards darkness, but her arc most certainly didn't go in that direction. On the contrary, it became Angel teaching Darla what love really was and saving her soul in the process, ending with her redeeming herself twice in acts of love. Dangel is NOT Spuffy. It's not even Sprusilla.
And no, soulless Spike and Drusilla aren't capable of real selfless love (agape). Jealousy and affection, as the Judge put it.
With Drusilla, she runs to protectors who punish her for her perceived sins. She's stuck in that mode because of her abusively religious upbringing by her mother (blamed for her visions as a sign from God), which is what Angelus used to break her. Angelus latched onto Drusilla's mother situation when he told her to do those acts of contrition, followed by her believing she needs to be punished all the time before she will receive love. Spike even knows this when he says that in order to win her back from the Chaos demon, he's going to find her and torture her. Drusilla believes that punishment = love. Also, as a particularly fragile vampire, Drusilla needs and wants others to protect and care for her.
On soulless Spike's end, it's all about obsession and getting something he wants with little care for the other's feelings or well-being. It's always selfish with him. With Spike's mother, this storge mania is actually quite malignant to start with, but the human William would never have thought vampirism and damnation to the be same thing as 'saving' Anne Pratt by turning her into something that no longer is even her. That's the demonic vampire's twisting of leftover memories and emotions into its own form of procreation and survival (Jossverse vampires are nothing but animalistic predators in their pure form). This is no different than Darla telling freshly-risen Angelus that killing Kathy, the symbol of Liam's only non-eros feelings of love (storge), was an act of love. Angelus then becomes the enemy of love.
But nobody would accuse Angelus of not continuing to feel eros, just as Darla saw it as her former profession and thus an expert. Darla also saw Angelus as her great protector because of her extreme fear of death (just as the Master was when she couldn't have Angelus, who also provided her with "the view"). She desired for him to fulfill the love she had never had, yet didn't really understand how it wasn't real love because both were incapable. She was so desperate to prove that she was loved that she even tried to get soulful Angel back twice by making him prove it. Only her desire to be loved overruled her sense of self-preservation (which is why she left herself wide-open to Angel when he killed her for Buffy, followed by her eventual redemptive sacrifices).
No, ensouled Darla and Angel (as well as Spike when he's not listening to Buffy's delusional garbage like that comic) are much better experts on the subject of what vampires are capable of in terms of love. Angel, in particular, is an expert in agape and Darla also comes to understand it and act on it. Soulless Spike and Drusilla are highly unreliable on the subject. Vampires understand the likes of eros, pragma and mania, even storge and philia on some level that are usually leftovers from the human host, but never agape.
- Eros: Love of the body. The first kind of love was eros, named after the Greek god of fertility, and it represented the idea of sexual passion and desire.
- Philia: Love of the mind. Philia, or deep friendship. Also know as brotherly love, philia represents the sincere and platonic love.
- Ludus: Playful love. Ludus refers to playful love where there is no commitment.
- Pragma: Longstanding love. Pragmatic, practical, mutually beneficial relationship.
- Agape: Love of the soul. The highest form of love, charity. Universal, unconditional love that transcends and persists regardless of circumstance.
- Philautia: Love of the self.
- Mania: Obsessive love.
- Storge: Love of the child. Means "love, affection" and especially of parents and children.
