That is, in this book, Light's ruthlessness is short-term, and of necessity (they deeply regret harm). So a death may be required, but they hope or try to avoid it to the extent their mission permits. The Dark is at best indifferent to long-term suffering and torment.
As I noted in a previous post:
by the end of the novel, Will is no longer a child in any meaningful way -- he has come into his own as an Old One and has cosmic responsibilities beyond his family's ken. Indeed, Merriman admonishes him to "remember yourself. You are no longer a small boy" (III, "The Joining of the Signs" 288). Will's separation from family is sharp and painful. Before the story ends, he wipes Paul's memory (II, "Christmas Day" 180) and is prepared to sacrifice his foolish sister Mary (III, "The King of Fire and Water" 256). Perhaps the worst moment is when Will considers the qualities that make his brother Paul uniquely worthy of trust, and decides not to confide in him (III, "The King of Fire and Water" 238). Needless to say, Will has quickly learned to deceive and manipulate his relatives to protect his mission; his love for them, like Merriman's for Hawkin, has been subordinated to the battle against the Dark.
Other manipulations of his family include:
- freezing them in time - all of them, in their own home (III, "Christmas Day" 160) and
- "clos[ing] off the minds of his brother [Paul] and the rector behind a barrier that no power of any kind could break through," even though it would leave them "like vegetables, incapable of any communication, forever" if anything happened to him (id. 174).
These actions are motivated by a desire to protect his family; and the unexamined assumption that he, unseely-Will, knows better than they what is best for them.