Radovid is actually crazy at this point. I agree that the scene is wonderful, but I'm not sure is wonderful in the way you're thinking.
I wouldn't say he is crazy. It woud mean he doesn't know what he's doing, or that his decisions are completely irrational.
He has a motive for everything he is doing.
I would compare him to Hitler.
Obsessed with hatred toward a certain minority (which eventually constituted to his downfall), amoral but a tactical and strategical genious, able to outplay everyone, or almost everyone else.
He definately was paranoid, he saw enemies everywhere and was almost blind with hatred, but he had a reason for everything he was doing.
And he had a reason for his paranoia. People were trying to kill him. Filippa was out for his head. Sorceresses were still conspiring against him.
I do wonder about his logic regarding the invasion of Kaedwen. He probably saw the oppertunity for a power grab and decided to gain more land so that he could divide the north between Nilfgaard and himself.
I wouldn't say he considered dividing the North. He wanted to push Nilfgaard back, and become a sole ruler of the North. As Roche says, he promised restoring the old borders after winning the war - but he definately wouldn't do that.
Without him grabbing Keadwen, the North would to too divided to fight Nilfgaard. Temeria and Aedirn were already lost. Keadwen alone would be no match for Nilfgaard, and then Radowid would be left weak and alone. This way he created one strong country in place of two - and used the time to consolidate his forces further. With him being the sole ruler in the North, he hoped that all remaining forces would fight for him.
Someone compared him to King Aerys II from GoT - but Aerys was stupid. Radowid had all the insane cruelty and paranoia of Aerys - but the mind of Aegon the Conqueror.