After his mask, his soul, had been ripped away, Angelus grabbed Xander in the hallway. He'd noticed what none of the others had: underneath that bumbling exterior, Xander actually had a brain. And Xander hated him, which, unbelievably, made him Angelus' greatest threat, after Buffy.
And she was special. He was saving her for last.
At his command, Willow ran off to the library. Leaving Xander alone with him. Giving him time to drag Xander to an abandoned mansion he'd spotted, up on Crawford Street, before anyone recognized the threat.
They wouldn't be found there. Not in time.