I found the fic. Life Hard - Spike re-living his life from human to vampire with different choices, starts in chapter 16.
I remember this scene.
Darla tells Angelus that Willaim is soft, tender, he makes love, not f**** like a demon, he's making Angelus care, Darla goes away somewhere and Angelus tortures William, William becomes catatonic, Angelus wants to stake him, but Darla arrives in time and stops him, Darla f**** William and threatens to stake him if he doesn't come back, Willaim snaps out of it and f**** Darla to prove he's a demon, William changes his name to Spike, |