My daughter is 14, and my wife and I have always placed an emphasis on nurturing her independence by allowing her to play alone with friends in the same way that we used to when we were kids. We encourage her to be social with people she has never met (although, "never go anywhere with a stranger" is still a good rule to live by, and one of the few generalizations we asked her to live by), and she's learned a lot about how to interact with various personality types as a result.
She's an extremely well-balanced and happy kid, making excellent decisions when confronted with things like sex and drugs, and the only real credit we can take as parents is having the good sense to know when to back off (which is frequently).
What happened to Maddy is a largely unavoidable tragedy similar to Sandy Hook and the Aurora theater massacre. Instead of obsessing over how we can avoid being victims of random twists of fate, we must shift our focus to figuring out how our society can better identify and deal with the maladaptive minds that perpetrate these horrible acts.
TL;DR- Saying that parents should be responsible for their children not being murdered by (extremely rare) psychopaths is not that different from saying women should avoid wearing revealing clothing if they don't want to be raped. "Stranger Danger" is victim blaming, and it needs to end.
Statistically, children are over 10x safer with strangers than with their parents.
Source:
http://www.bjs.gov/index.cfm?ty=pbse&sid=31