No, Hollyleaf does. It may seem like Squirrelflight did but she didn't 'cause Hollyleaf admits that she did kill Ashfur in Sunrise.
No, Hollyleaf did.
Ashfur did not kill Squirrelflight in any book. He did attempt to harm her in "Sunset" (Warriors: The New Prophecy series) by attempting to push her off a cliff, but Squirrelflight survived. Ashfur's actions were driven by his jealousy and betrayal, but they did not lead to Squirrelflight's death.
Ashfur
No, it was Ashfur.
*Spoiler Alert* No Hollyleaf does.
No, Ashfur would never try to kill Squirrelfight. He loved her but tried to kill Hollyleaf, jayfeather, and Lionblaze to get revenge on her. Then she revealed they were not her children and he didn't kill them.
Hollyleaf convinced Ashfur to kill Squirrelflight's kits by revealing the secret of their true parentage. This revelation led Ashfur to seek revenge against Squirrelflight for lying about who their father really was.
Hollyleaf killed Ashfur and attempted to kill Leafpool.
Ashfur tried to kill Squirrelflight, Jayfeather, Hollyleaf and Lionblaze because he wanted Squirrelflight to feel the pain and anguish that he had felt when she rejected him to become his mate and when she supposedly had kits with Brambleclaw.
Well, sometime they have to figure it out.
No, she killed Ashfur and she's dead now, so I don't think she will kill Firestar.
Yes she does then puts his body in the stream next to the Windclan border.