This Boy's Life by Tobias Wolff is an autobiography. As such, we cannot read it as an objective document or even a literary work, but rather as the effort of an author to create a self-image for a reading public.
As Wolff describes being raised a Roman Catholic, and the act of Confession is important in the book, we should probably read this as a confessional narrative in a genre pioneered by St. Augustine. The point of Confession, in part, is to recall and repent every possible sin one has committed, in order to be sure of doing the appropriate penances and being granted absolution. One standard topos of Roman Catholic coming-of-age stories is inventing sins under the pressure to conform to the formal pattern of Confession. A second impetus for self-fashioning to emphasize his own misdeeds would be establishing what in popular culture is sometimes called "street cred"; it establishes as aura of authenticity as someone in touch with a grittier life than the reality of his upper middle class prep school background and family might suggest.
Whether Toby is a good person is not really something we can judge; within Wolff's theological system, only God would be able to judge that. As a literary character, he falls into the "lovable bad boy" trope, disarming criticism by means of self-critique. Certainly he is a badly behaved teenager, but that is a phase many teens go through.
In reality, there are very few people who are all good or all bad. Toby is not a mass murderer or something irredeemably evil, nor a saint, but a troubled adolescent, who over the course of the book seems to be going through a rebellious phase, lying, cheating, and having difficulty connecting with peers and family. As Wolff is now a successful author and professor, he has probably developed better impulse control, but without being able to see into his heart and soul, we cannot judge his ultimate moral character.