You're asking me what her motive was ? Haven't you read anything about WHF at all ?
I wonder what you're going to put forward. She wasn't mercenary. She adored her boys. She adored her father. Her relationship with June was iffy but I imagine Nevill would have been somewhat put out if she'd killed her. Clear minded and sane, I can see no reason for her to kill. Clear minded and sane, I can see no reason for her to plan to kill. Deeply depressed, as she appeared to be during her last fortnight, she'd have lacked the mental energy required to kill.