The real reason Sherlock deleted the solar system was because when as a child Mycroft explained the concept of black holes to him, Sherlock had a nightmare of being sucked into that empty darkness. He wasn't afraid of dying, he was afraid of being forgotten and erased.
A/N: Friendly reminder that he over powered this fear for John, sacrificing his dignity and his frailty as a detective. He conquered them all because of JOHN and his friends.