What is the "real bad" that could happen? Until we define that, hard to answer.
The list of things that
could happen is endless.
Sure, but that list exists with everyday life.
In all honesty, I see 3 things that would give the most cause for concern.
1) Getting hit by a car/accident.
2) Wandering off/getting lost
3) Getting abducted.
Seriously, what else is legitimately going to happen walking ONE block home?
As for getting abducted, the chances of that are so ridiculously small, especially given the distance covered here, that it's not something I would worry about. Yes, we hear about stories, but that's more an issue of the availability of media reporting than being a real danger that should be factored into this decision. If that is truly a reason driving a decision, then you shouldn't let your child swim alone, bathe alone, play on a playground, eat alone, or really do anything alone as well as put them in a car, etc.
As for wandering off, getting lost, or getting hit by a car, that is why I said above that it depends on your 6 year old. If you have a "mature" six year old that those items aren't a concern, then I see no issue with walking. If they are remotely a concern, then I don't see any way you allow them to walk alone.
To me, it's a pretty simple question that a parent should be able to answer about their kid. Then it's a matter of their comfort level. But I think there are certainly 6 year olds that are capable of walking one block unattended.
ETA--In 1999, there were 115 "stereotypical kidnappings" across the nation for the entire year. This is out of 72 million children. You can feel free to base your decisions on those numbers but I like to be a little more realistic about the actual threat.