Why would he want a child to hang around, even a vampiric child?
I would have loved though if Angel's sister had been sired.
That would have been awesome.
When Angelus decided to sire Drusilla, he didn't know her personally, at least the human-Liam (neither did Angelus) had not known the human-Drusilla personally. She was not his neighbor, girlfriend, close-friend, sister, cousin or whatever. Angelus had heard about Drusilla's powers of foresight, her purity, her chastity and sweetness of her character through Darla. But other than that he didn't have any personal connection or attachment to her. So when he decided to take up Drusilla as his obsessive pet-project, he didn't actually know her personally. As far as Angelus was concerned, Drusilla was a stranger to him.
But if Angelus had decided to sire his sister, he most probably would have tortured her in ways that were ten-twenty times worse than what he did to Drusilla. Because unlike his relationship with Drusilla (or the lack of one, before he had turned her into a vampire), his relationship with his sister had some personal connection. In other words, siring his sister wouldn't be just some obsessive-pet project, but something that he would have taken very personally.
As I said earlier, the human-Liam seemed to be quite close to his sister. If you notice in the flashback scenes in
"The Prodigal" , when Liam is about to leave his house once and for all, he bids a tearful and intense goodbye to his sister, but he doesn't even say anything to his mom and of course his dad gets all the harsh words. To me, it seemed like he was really going to miss his sister. From that I can make a pretty good guess that he and his sister shared a very close-relationship and his sister was most probably the only family member who understood Liam and was a good friend to him, and he in turn loved and adored his baby sister. His sister most probably inspired feelings of love, affection and adoration in her big-brother's heart towards herself.
My point is: Love, affection and adoration are VERY much human emotions and feelings which Angelus hates and despises. Human emotions and feelings (like love) is everything that Angelus stands against.
Remember in
Buffy The Vampire Slayer season 2 episode,
"Innocence", when Spike asks him why he is so hell-bent on hurting Buffy, to which Angelus replies:
Angelus: She made me feel like a human being. That's not the kind of thing you just forgive.
So if he wanted to sire his sister, he would have taken it quite personally, because she made him feel human, in other words, she made him (the human Liam) feel emotions like love towards herself. If he had decided to sire his kid-sister, he most probably would have harassed, humiliated, tormented, tortured, abused (and yes, even sexual abuse) and eventually rape her (i feel sick to my stomach as i type this, but this is Angelus we are talking about. He is a psychopathic, sadistic bastard who is capable of doing anything) and then finally sire her. It's his way of twisting, warping, poisoning and ultimately destroying any real love that the human-Liam had for his sister.
Now that I think of it, I'm actually glad that he killed her, because in cases like this, death is a sweet release from any pain, suffering and torment that his little sister would have had to undergo.
