score:6
Detroit's Devil's Night, was, generally speaking, more about arson and fires, than vandalism and personal injury and death. Be aware that the city has been combating what is now called "Angel's Night" since the mid-90's and for most of the early 00's, the arson levels were near normal. There was a slight increase in 2010 and 2011, but for the last two years, the damage has been, relatively speaking, light.
As far as the concert, just be aw wary and aware as if attending any other big-city large-crowd event. Don't flaunt wealth, stick to lighted areas as much as possible, and go with the crowd.
Detroit's population is only fraction of what it was even two decades ago. There are fewer total people (around 700,000 total, as of 2012), which, of course, means less taxes, which means less police and fire departments. Those that are left, however, seem to be more likely to go along and get along than ever before.
You'll notice, as you read this, that I've not linked any articles. If you do a Google/Bing search for "Detroit Devil's Night" and "Detroit Population Change", however, you'll find many useful articles corroborating my statements. The mass media popularizes the bad things, but then, it's a city, and crime happens. As you are specifically asking about Angel's Night (or Devil's Night as it used to be referred), it's not the fire-filled event it used to be.