Picks up during S7, 'The Killer in Me', where Spike is in tremendous pain from the misfiring chip and Buffy is frantically trying to help him. 

They go together to the Initiative... however, the soldiers do NOT show up when they do in canon. And in her desperation to help end Spike's suffering, she at least contemplates a mercy kill. 

The contemplation is the only real must here... notice I didn't say she actually kills him. (However, if that's the brave path you choose to take, so be it!) What's interesting to me about this is the feelings that are awakened in her as she considers having to live without him. It could be something she thinks about on her own, or he could beg her to do it and she refuses, or she could try and fail... Take it wherever you wish from there! 

Maneuvering out of this scenario is totally up to the author. Maybe the Initiative Soldiers still show up, just later? Maybe she begs Willow for magical help? Maybe she strikes a deal with some other demon to somehow get the chip removed? Maybe she time travels and fixes things, somehow? You do you!