Yeah... seeing one of his friends almost die was Yoshiki's breaking point, coupled with the fact that he found out 'Hikaru' probably killed that old lady. He was keeping around a monster that replaced his friend because just the mere presence of his deceased friend's face was a comfort. Its sad...
I just noticed something. Whenever 'Hikaru' starts to unravel his form, usually we see it begin in his left eye, which is the same eye that the real Hikaru had blood leaking over. Don't know if it means anything, but thought it was neat. (Small theory but maybe Lord Brainsnatcher entered Hikaru...