Why does Billy pay for Martha Timberwolf's funeral in the novel, The Whites?

When Billy learns that Martha Timberwolf has been stabbed to death by Shakira, who is the sister of Memori Williams (a victim of Curtis Taft), Billy feels personally responsible for not being able to put away Taft. As a symbolic act of respect and remorse, Billy fronts the entire cost of the Timberwolfs’ funeral expenses.