Jealousy.
She was probably better than him and more well liked.
I'm interested in knowing how long he worked there.
A lot of stupid people don't actually realise they're stupid. They don't realise that they're being carried and assigned work that is at their level; there's no shame in this and I'm speaking harshly because of how the situation turned out.
My guess is he's seen countless people over the years move ahead of him. It probably rubs him the wrong way each time because he's unaware of his limits. Seeing a young woman outpace him was just too much.
If the death penalty is on the table, I hope she gets it.
If there's life after death, I hope she finds peace. A tragic and unnecessary end to a young life.