This is scary, but it is a form of OCD/intrusive thoughts. The fact that thinking about these things upsets her is a way to know that she would not actually do them. Everyone has intrusive thoughts but most people are able to dismiss them without much thought. Some kids just get stuck on them.
Here is a good article:
http://www.worrywisekids.org/node/120
One of my kids has this. She started talking about suicide and being afraid she would hurt herself and she was terrified. Also about bad or violent things happening or the rest of us. We have had huge success with cognitive behavioral therapy for anxiety (OCD is a form of anxiety). It took several tries to get a therapist that really helped though. She also takes anxiety medication which was the only way she would even begin to use the skills she was learning in therapy. (I wouldn't start there, but it was the combination that worked for is.)
I don't think it will go away on its own. Her baseline anxiety might be lower when schools out which will help but in our case, it can get really bad too which has resulted in severe panic attacks. It's tough to come back from that point even with medication.