Life is cruel, darling, but what you could do for them was to become a good king for their people.

( Jesper doesn't know if Nikolai's father was a good king. His guess would be not, considering what the Grisha went through, he's sure that Nikolai is better. He has no opinion of his brother. But that seems like the kind of thing that royals should care about; being good at it, taking care of others. What Nikolai actually cares about himself, and does well.)

And you did get rid of the Darkling eventually, yeah? So they were avenged.