How did the findings about Keller and his wife explain the sort of person he was?

The most important part of Paul’s trip to Adelaide is not his performance in either of the competitions, but the moment when he sees Keller’s tattoo. He realizes for the first time that his teacher had been in a concentration camp and experienced much more suffering than he realized. Paul has come a long way from the beginning of the novel when he joked to his parents that Keller might have been a Nazi war criminal.

Keller's guilt over the loss of his wife and son and being unable to save them is a driving factor in his life. Because of his guilt and regret, Keller erases his former identity and goes into self-imposed exile in Darwin. He renounces the emotional music he loved the most and amputates his little finger to prevent himself from playing the instrument that he was playing while his wife and son were captured. Although the Crabbes and the Wallaces try to feed him German and Austrian food and talk to him about his homeland, his anger and regret over what happened there makes him say that nothing could ever make him homesick. Paul also experiences his own form of regret for not spending more time with Keller. In hindsight, he wishes he had not given into the temptation to spend time with his girlfriend instead of with Keller. He wishes he had written to and visited Keller more and spent more time learning from his maestro before it was too late.