It started as a fleeting thought.
Brambleclaw yowled at poor Squirrelflight, he had thought. Someone should empathise with her.
Nobody empathised with him when Brindleface was murdered. He had to make sure that never happened to Squirrelflight, so she wouldn't suffer what he had once suffered.
It turned into a relationship.
They met in the moonlight, they hunted and patrolled together. He ignored Brambleclaw's despondent looks.
It wasn't meant to be.
She abandoned him for the one who caused her all the pain, because love was hard to control.
That fire was like the one raging inside him, the one she began, backstabbing him the way she did. It charred his heart black, black enough to perform the callous deeds he did.
It changed him.
In each kit, he saw his desire, and he saw it crushed. Those kits were to be crushed too. Or incinerated in the way his feelings were, or burnt out in the way his spirit was.
It hungered him.
Empathy was something he never had. But this way, whether she liked it or not, she would know the pain she inflicted on him. He would force her to empathise.
It killed him.