It's the way he'd have wanted to go out, really. Fighting fiercely. Dueling Bellatrix Lestrange, the cousin he always thought was redeemable. Until she spent even longer than he did in Azkaban, without anything to hold on to.
She went completely of the deep end.
She's now lost completely.
She cackles insanely, and Sirius is reminded of himself, as they carted him off to Azkaban. He laughed like that, too. Maybe he shouldn't have. Maybe that wasn't the best choice.
Still, she's lost in a way that he never was.
This doesn't negate the danger. Crazy or not, she's still dangerous.
He still can't believe it when she wins. In his last instant, his last realization is that he's lost.