^^ I don't think Murray has allowed the tragedy he endured/ survived in his childhood (in the school shooting incident) to define him as a person or as a tennis player. And "turn into a decent person," is not the best choice of words, IMO, but I understand you seem to mean he grew up to be a productive human being. That would probably have been the case regardless due to his family and to the upbringing he had. I think it's wonderful that Andrea Jaeger is one of the people responsible for making an important impact on Murray's life after the tragedy. I believe the opportunity he received to concentrate on and further develop his talents in tennis was quite helpful to Murray in the aftermath of such a traumatic experience.
Sure Murray used to display negativity on the court, as have a great many players, which probably has more to do with personality quirks and/ or bad habits. Murray's negativity has usually been mainly against himself for his poor play and it has never worked to his advantage. His current coach, Lendl, has largely been able to wean Murray away from such self-harming patterns of behavior on court. It seems to be rooted in negative thinking, which generally doesn't work for anyone. John McEnroe was the kind of player who used angry emotions to rev himself up in a positive fashion, but how that seemed to work for McEnroe is definitely an anomaly.
Lendl is another great player who I've always admired. Lendl knew himself and he understood how hard he needed to work in order to succeed because he was not as naturally talented as some of his opponents. He also possessed a great deal of deadpan wit which tended to go over the heads of the media. McEnroe hated Lendl during their playing days, but has come to respect and admire his greatness as a champion.