Dawn had been the one to call him. She was crying and begging him to stop Buffy from ruining her life. He'd raced to the police station, desperate to get there before she did, sighing in relief when he heard fists hitting flesh.
Buffy's hitting out her rage on something, someone, he hesitates.
"You don't ... have a soul! There is nothing good or clean in you. You are dead inside! You can't feel anything real! I could never ... be your girl!"
Spike just lays there, taking it and all Xander sees is his mother under his fathers fists.
"You're stupid, a moron, how dare you stand in my way!"
Xander can hear his fathers every single word through the paper thin walls. And all too soon there'd be that little sound where his mom was trying to hold in her sobs, even as she never fought back.
He was never able to protect her, or be there for her.
He doesn't even think of the danger to himself as he grabs Buffy's fist, gets thrown to the wall for his trouble. Her face turns to horror as she sees him.
"You don't hurt the people who love you."