Welcome to SR, Jen.
I am so glad you are taking this seriously, as sad as it is. Active addicts are not predictable and often have mental health issues that need to be addressed. Problem is, when they are actively using they cannot be assessed for the mental health problems.
At this point it doesn't matter why she is like she is, what matters is the safety of your father and yourself and family. Believe what she wrote, even if it was a passing thought during a dark moment. It could happen again and she may carry through.
We are all here to support you and walk with you through this difficult time.
Keeping you and your family in my prayers.
Hugs