Mortal: Who was Drusilla before she was a vampire?
"Drusilla was a young cockney lass whose family were coal miners. She was Catholic and lived in London." —Joss Whedon
She was born in mid-1830's. In 'Becoming' we see her just as Angel described her: a pure, sweet, chaste girl. She already has her ESP powers—only now, she doesn't revel in them. Quite the opposite. She sees them as a sign from the devil and is convinced they make her evil.
In 1860 Darla finds Drusilla and shows her to Angelus, offering her as a 'gift'.
He takes great interest in her, first thinking she's a saint, then realizing that she has the sight. He thinks that's much better as she will see what's coming, but be unable to stop it. Drusilla turns around and sees the vampires stalking her. While she may not knows they are vampires, she knows they are evil and protectively rushes her sisters along.
In 'Becoming', Angelus is in the confessional killing the priest, but he is obviously surprised when the object of his latest obsession shows up there. Drusilla's visions let her down; she doesn't know that the real priest is dead. Angelus uses the unintentional encounter to his benefit, telling her she *is* a Devil child. He belittles her God but tells her, nonetheless, to fulfill his plan. To give in and be evil.
Drusilla begins to cry. She doesn't want to be a creature of evil, and begs forgiveness. Angelus reluctantly gives it to her.
In this scene we also get the impression that she goes to confessional often. In 'Becoming' it's only been two days since her last visit to the priest and she's a shaking, terrified mess.
We later get a glimpse of mortal Drusilla once Angelus is done tormenting her with the head games and deaths. Or so it seems. She's a complete wreck, huddled in the far corner of a church.
Angelus has killed her family, friends and everyone else she might have loved, as well as called upon Darla to help him slaughter the nuns in the convent where Dru had fled to take her holy vows.
When he informs Darla that he doesn't plan to kill Dru, but to make her a vampire, even Darla is stunned by his sadism. She protests, but Angelus has made up his mind, wanting to preserve his work as well as her suffering.