After Cash App founder Bob Lee died in a stabbing, some were quick to blame San Francisco’s rising crime rates. But prosecutors say Lee knew his alleged killer. WSJ’s Kirsten Grind reports on an underground world of sex and drugs that was the backdrop to the killing that shocked the tech community.  Further Reading: - Before His Killing, Tech Executive Bob Lee Led an Underground Life of Sex and Drugs  - Cash App Founder Bob Lee Fatally Stabbed in San Francisco  Learn more about your ad choices. Visit megaphone.fm/adchoices