"Your victory over him took but moments. But his defeat of you will last lifetimes."
It's interesting to think about Angel's character in light of Darla's explanation about how by murdering his father, his father will always have power over him. It also sheds light on why he destroys his victims (he doesn't just kill them, he destroys them). Drusilla is his greatest work of art -- she's in his power, his creation.
In light of Buffy, I've always seen Angel as discovering her when he'd given up any desire to control and shape other people. He saw her as the authority over him (The Slayer) and her innocent love for him (virgin girl) as that which would save him. Buffy is forgiveness, his way back into the light.
I think Angel craves and needs forgiveness far more than he wants love. That's why Buffy's love isn't enough to save him in "Amends", because she cannot pardon him for his crimes as he once naively hoped for in Season 2. He's been schooled otherwise; after a hundred years suffering in hell, his past victims still haunt him.
What's interesting is that when Angel loses his soul, he loses his ability to love. His love turns to hate, two sides of the same coin. But for Angel, I think it's less about love and more about power. Angel with a soul wanted to be in Buffy's power because being her "good dog [who doesn't] bite" means that he is good. When he loses his soul, he doesn't want to be in her power, he despises all that she stands for and all that he wanted her to bestow upon him -- good and innocence -- and so he despises her. She made him feel like "a human being", she made him feel powerless. And "that's not just something you forget", so he's desperate to destroy her, to make her his creation, to have power over her so that she can never have power over him again. This comes from deep insecurities. If Angel were actually confident in his power, he wouldn't need to devote his soulless existence to tearing others down.
Angel doesn't feel his power is innate. He's a gutter rat, a puppet, chosen by the fates and higher powers. He's the dissolute son who never pleased his father and his father's disapproval made him feel like he had no sense of worth -- of course, he probably became a dissolute wastrel because his father never allowed Liam to feel a sense of worth, so Liam sought comfort in alcohol and women.
Love by itself is never enough for Angel; it always has to mean more than just being loved because what Angel has always wanted is absolution (with a soul, he wishes to be absolved from his sins and find forgiveness, without a soul he wishes to be absolved from the chains that bound him and his feelings of powerlessness). As Liam and Angelus, he desires absolution so as to be set free from obligation. As Angel, he wishes to be set free from his guilt, to finally be forgiven. But the prodigal son can never return home and be forgiven.
Darla was a way to say F.U. to his father--instead of settling down and rearing a respectable family, he's forever a wastrel, forever drinking (blood) and sleeping with prostitutes (Darla). When his soul returns, his feelings of self-admonition come crashing down (does that voice that recants his crimes in his head sound like his father?). Liam probably felt guilty when he was drinking beer and refusing to marry a good woman because he did care what his father thought of him. To be confronted with his ability to care for what he as Angelus did to his father, his sister, to thousands of innocent people -- Angel spending all that time in the gutter is the extreme of Liam spending all that time in bars drinking and sleeping around to numb the pain of his failure to be good. Only for Angel, the guilt amplifies to the point that he's unable to escape it no matter where he runs or how hard he hides.
And then he sees Buffy. Buffy was a way to be forgiven, to feel human, to be free of guilt -- and he does achieve a moment of perfect happiness. For one brief and shining moment, he finds absolution. He feels free of guilt, he knows happiness and acceptance -- he couldn't find it with his father, so he sought it by taking a good 'wife'.
Angel is ever the Catholic, seeking forgiveness from higher powers (first Buffy as The Slayer, then the PTB) and wishing for a concrete form of penance, to pay his dues; eventually he realizes it'll never be enough, but I think even when he knows this, a part of him still hopes for it, hopes for some authority to cleanse him. He doesn't seem able to accept his sins, let them go, and move forward to do good -- and considering the monumental nature of his sins, that's not surprising.
His souled journey then begins with him seeking Darla who symbolized his ultimate freedom from guilt (the guilt she first freed him from when she turned him, the guilt of never being enough for his father), but Darla isn't enough. Then after a century of misery, he sees Buffy as a source of forgiveness (and for a moment he is free), but because of the curse that means he must never stop feeling guilt, she's not enough (she is no longer his savior, but his temptation: "I want to take comfort in you, and I know it'll cost me my soul, and a part of me doesn't care"). Then it's the PTB as a source of forgiveness (because of the curse now forgiveness can only come when he's human), but they too are not enough.
As a vampire with a soul, burdened with a guilt that he can never let go of (because letting go of guilt for his crimes means allowing himself to be happy and then he'd lose his soul), Angel can never find release through the absolution he craves (well, he could, but being released from guilt means becoming a soulless monster), he cannot be forgiven by being powerless in the face of a greater authority (becoming a soulless monster wouldn't bring him forgiveness but damnation). After Jasmine, Angel's faith is shaken to the point that he disbelieves a higher power will forgive him. Thinking he cannot be restored to goodness by higher authorities, he then accepts further damnation by agreeing to be CEO of W&H if a good will come of it for others.
We'll see how he no longer believes in the Shanshu and in forsaking higher authorities, he will accept that he is the only authority. No longer seeking forgiveness in powerlessness, he again takes a hold of power over others (remaking Connor, remaking his friends memories), and the line between Angel and Angelus blurs even more, to the point that his own team worries that he's lost his soul based on his behavior. Angel in "Power Play" and "Not Fade Away" has fully embraced his power to destroy, not merely in battle as an honorable warrior and champion of the Powers, but as a destroyer who can slip inside, poison your meal, knife you in the back, and even turn you against yourself through mindgames and manipulations. We see Angel fully employing his favored soulless methods which he tried to disavow. The end of Season 5 shows Angel using his potential for manipulative ruthlessness to do good, using his skills honed while soulless to save the world. Necessary evil for a greater good. He's playing the part the way he played Angelus in "Enemies". The ruse begins just with fooling Faith (and Xander); he'll play the part of soulless most notably again at the end of AtS Season 5 in "Power Play"; and he'll continue to play the part of soulless in Season 8, wearing the mask for so long and to such an extreme that he completely loses all sense of self.
Angel in Season 8 turned his back on his soul for so long that he lost all perspective. He found a new way to, if not be free from guilt, then to distance himself from it -- he can rationalize away his crimes by telling himself that they're necessary for the greater good. And by saving the world and saving Buffy, he may even be able to rationalize away all his past crimes in believing that this is the moment he was meant for, this is his destiny, that he's the only one who can do this and that he learned these skills as Angelus in order to use them as Angel -- he committed centuries of rape, murder and torture so that he could use them now to save the world from destruction. Darla damned him initially, but in Season 8 we see Angel damning himself in the hopes for an ultimate forgiveness. He can't go up in search of forgiveness, because of his curse he cannot go up (going up means one step forward, then because of the curse he's jettisoned back to complete soullessness), so paradoxically he rationalizes that he'll dig himself in deeper, thinking the way out is by going further down.
And this is Angel's tragedy in Season 8: When the necessary evil Angel has dedicated himself to wielding is revealed to be not for the greater good, but simply for more evil, his rationalizations shatter, leaving him near comatose. His actions in Season 8 were a last-ditch effort after years of trying to find absolution (from Darla, from Buffy, from Darla again, from the PTB) and continuing to fail. He's forever seeking relief from his sins and his crippling guilt, but he can't get no satisfaction.