In that fateful alley, in the warm breast of Darla, Liam died and found new life as the vampire Angelus. After the gypsy curse restored his soul, he was tormented by guilt, led a life of self-pity and repentance till he reinvented himself as Angel and began to build a life around atoning for his sinful days. Angelus is evil, Angel is good. They are, still, the same person. So, divesting his personality of goodness or evil, what are the characteristics of the undead Liam?
The most salient feature I see, is his intense desire to fit in. Towards that end, he is passionate, dedicated and inventive: In his apalling acts of cruelty as Angelus, and in awe-inspiring acts of sacrifice and compassion as Angel. We see it in the murder of Jenny Calendar (Passions), and in his creation of Drusilla (Becoming, Part I), and in flashbacks of Holtz's family (Lullaby). We see it again as he risks his life to win a second chance for Darla (The Trial), even as he is not in love with her, nor has any reason to believe that she is more worthy of life than he. Similarly, he refuses to give up on other troubled souls, no matter how misguided their actions, a compassion he shows with Faith (Sanctuary), and in later years, Connor (Home).
What drives him? It may seem that Angelus was always striving to gain approval from Darla, striving to surpass himself in depravity in order to prove that he was worthy. In his later incarnation, he shows the same drive in seeking to impress Buffy. Love? Perhaps; perhaps a more basic need than that, for his traits remained even after Darla (to Angelus) and Buffy (to Angel) were out of his life. Perhaps a need to simply be the best that he could be, in order to prove to himself that he is worthy. As Angelus, worthy of fitting in as a vampire; as Angel, worthy of fitting in as a souled being.
The gypsy curse ensured that Angelus could no longer fit in as a vampire. The Shanshu prophecy gave him hope that he would eventually be able to fit in as human. After years of suffering and heroism, when Spike challenges his right to the prophecy (Destiny), we're suddenly presented with a stark reminder of who Angel really is: Angelus. Spike is the only character who sees this truth every time he looks at Angel. However sympathetic the viewer may have become to Angel's fight for redemption, Spike is the only one able to look at him and say, "So what?". With good reason. He has seen Angelus longer than he has seen Angel; which is longer than we as the audience, or any of the human characters, have seen either of them. He is the only one who can judge him as a peer and a fellow victim of vampirehood. Always the underdog to Angel, through his eyes we don't see a tortured soul fighting for redemption, we see Liam as he lived after his death - as Angelus, as Angel, as one personality - in just one more attempt to end up on top.
Its that desire to end up on top that makes both superheros and supervillains. The flip of a switch, a tip of the balance, and you get one or the other.
In Orpheus, he has to choose which one lives. Blood is life - having drunk Faith's tainted blood, Angelus finds himself in battle with her, but on an unfamiliar battleground. They find themselves wandering in Angelís dreamscape and it is upto him to choose which spirit to align himself with. Whedon has previously touched upon the strength that comes from spririts enjoined (Buffy season 4: Primeval). This idea is revisited more subtly as Faith and Angelus play tug of war with Angelís soul in order to survive themselves. Faith needs Angel to continue fighting for good, while Angelus needs him to stop. Being soulless, there is no Angel manifest, so he cannot fight for himself. As Faith wins, though, we don't see Angelus blow up in a cloud of dust. We see him being engulfed by the body of Angel. Angel's spirit overpowers that of Angelus, but does not discard him. He cannot discard him, as good vs evil exist in an inseparable entity. A duality that can be thought of as fighting each other for supremacy, or simply as existing on a continuum, waiting for the balance to be tipped. Its hard to remember that Angel is the same person as Angelus, and when we're reminded, we don't want to believe it. But its the truth. Its undeniable. And till the very end, the Shanshu is left unfulfilled.
Images adapted from WB, Mutant Enemy. Written by greyspacesATagnibirdDOTcom. Last edited Mar. 14th 2006