Honestly, I think it was part suicide/part she wasn't thinking clearly. Dawn had pissed her off for the last time and she just wanted her dead. She could also simply have known that the scissors wouldn't be enough to kill Dawn and that the Dawn or one of the cops would take her out, ensuring that Dawn would die. She was never cut out for the apocalypse, and she was slowly losing hope. After seeing what was going on in the hospital and starting to kind of trust Dawn, only to see her pull her power trip BS at the end again, may have just been too much for her to bear. Remember, she tried to commit suicide once before, and was "brought back" by her hope and seeing how many people she had that cared about her. At this point though her father is definitely dead (murdered in front of her) as well as her not knowing where Maggie is. I can see her just being done with everything and not caring anymore.