Thom can see the future.
Alanna thinks that he can only do so intermittently, but she is working from a knowledge of Thom at ten, not Thom the Mithran master. Learning to control one's obvious talents is always the first step to training one's Gift.
He knows what awaits him in Corus. He knows that Alanna will slay Duke Roger, throwing her own honor into question and then salvaging it in one evening. He knows that she will then pull one of her usual hardheaded stunts and go off adventuring until she feels up to bearing the weight of the shield she carries.
He knows what Delia will ask of him long before he actually knows her name. She doesn't even need to ask him, really, but she doesn't know that and anyway, Thom needs her to be fully implicated in Roger's little conspiracy, or Jonathan will leave the emerald-eyed viper right in the heart of his court.
He knows what it will cost him, to resurrect Roger. But it is a choice between Roger now and Roger later, and Thom would rather have him eliminated for good as quickly as possible. Besides, if Roger is resurrected later, he will be even crazier, and will tear down the world around his head immediately instead of giving the others a fighting chance.
Thom thinks, briefly, that he should just leave it all alone, that he should let Delia find another, stupider, more easily dazzled mage bring Roger out of the Sleep. Mithros knows there are plenty of them hanging out around the palace. But Thom wouldn't wish that kind of slow, lingering death on anybody, except maybe Roger himself, and Thom knows that will never happen. Freeing the realm of the monster that is the undead Duke of Conté will require some sacrifices.
When he finally makes his way down to the catacombs, Thom's steps are firm.
Alanna is not the only one who has a sense of duty.
His last words to his sister are a lie.