Five is just . . . broken. Something's gone horribly wrong in his head, and now he's done something unbearable, and Turlough, the little fool, leaves him alone with it.

Watching Five let the Master burn was like watching Batman kill the Joker. Completely shocking.

I always interpreted the final scene with Turlough as the Doctor sending him away-- Turlough sounds like he wants to be talked out of leaving, and the Doctor interrupts him with that flatly final "I'll miss you," and Turlough accepts it and pretends it's his idea.