Living With The Guilt
In his defence, he hadn't initially realised. His mind had been elsewhere, on the murder of his father for one, so he hadn't taken in what he'd done when he'd co-opted an unwilling Detective Williams into being his partner.
He'd been caught up in the shouting, gun-waving, trading of insults and the general emergence of an attitude bigger than the State of New Jersey itself and so, he should probably forgive himself that he hadn't realised that the man he was about to repeatedly lead into harm's way was a father.
It had taken longer to realise that, for all the smart-mouthed retorts and declarations of hatred, that he'd actually come to like him. In fact, he'd buried under his skin and taken hold like a particularly insidious form of fungus, becoming first his partner, then his best friend, then the brother he'd never had.
This all comes back to him as he's sat in a moulded plastic chair at 4am, willing the unconscious man before him to wake up, and he wishes he'd realised sooner. Because then, he might have had a chance at blocking him out of his life. Then, he wouldn't have to live with his guilt.