Sherlock Holmes was getting madder and madder every year. He start consuming drugs because he couldnt run like old days so he needed drugs to keep running by all streets of London. Because of that drugs he start getting weird.
He also was getting mad because every case that he did, it drives him crazy for all that he needed to do for that case like running that much and for being in constant danger.
In my opinion Sherlock Holmes get so mad that he started to think about an imaginay criminal called Moriarty and that was a fight he couldnt win.He started killing people to give life to Moriarty.