In the book Tangerine by Eric bloor why does Paul wander into his backyard sick to his soul,what does that mean

Paul is sick to his soul because his brother Erik got into an argument with his friends who'd come to work on a science project at his home. Erik ending up taunting one of the boys (Tino), and then proceeded to beat him while Paul watched in horror. Paul is sick to his soul because he did nothing to stop it..... because he didn't know what he could have done to change the outcome.