Reasons why Sheila would stay alive for hours after killing her family:
She planned to kill everyone but not herself. But changed her mind hours later, just as the police moved in.
I can't think of any other reason.
If her plan was to commit murder/suicide, which supporters claim, then she would have killed herself straight after everyone else.
She was crazy, committed and brave enough to kill two adults and two children. So wouldn't have hesitated to kill herself afterwards.
If she didn't plan to kill herself why didn't she just give herself up when the police arrived and tried to communicate with her ?