Willow technically could've potentially killed Joyce in this episode. She sent Spike round to Buffy's house, probably thinking Buffy would be there, but she wasn't so Joyce was alone when Spike turned up. It's only because Buffy was on the phone to Joyce that she heard Spike's voice and headed straight home. Buffy obviously thought her mom was in danger hence her rush, and Spike retelling his tale of woe is what gave her the time to get there. However, if Spike had been a lot more kill happy, he could've killed Joyce and all because Willow told him to go to Buffy's house.
Willow might not have been aware that Spike had previously been invited (since she was unconscious for a while), and thus could enter the residence, but why risk it? It wouldn't be hard to wrangle an invitation if necessary (he got one off of Warren's mom somehow). Buffy could've been either at the library, at home or on patrol; Joyce was almost guaranteed to be at home. Why risk Joyce's neck when sending Spike to the library may have been the better option and at least stalled for time?
Obviously doing so gave us this scene:
But from an in-show perspective, it wasn't a smart move on Willow's part.