If you are interested in more of the ugly Welansky backstory...

Here is a documentary with a partial recreation of the events that lead to the prosecution of Barnett Welansky.  The introduction is a bit much, but the 20 minutes that follow give you a flavor of the story behind an historic and horrific event: 

In addition, if you think this could not happen again, consider that it roughly did (though with a smaller body count) in The Station nightclub fire in Rhode Island in 2003.  Here are the basic facts of that sad case and the legal aftermath (drawn from this Wikipedia entry):

The Station Nightclub fire was the fourth-deadliest nightclub fire in U.S. history, killing 100 people. The fire began at 11:07 PM EST, on Thursday, February 20, 2003, at The Station, a glam metal and rock and roll themed nightclub located at 211 Cowesett Avenue in West Warwick, Rhode Island.

The fire was caused by pyrotechnics set off by the tour manager of the evening's headlining band, Jack Russell's Great White, which ignited flammable sound insulation foam in the walls and ceilings surrounding the stage. A fast-moving fire engulfed the club in 5½ minutes. In addition to the 100 fatalities, some 230 people were injured and another 132 escaped uninjured....

In the days after the fire, there were considerable efforts to assign and avoid blame on the part of the band, the nightclub owners, the manufacturers and distributors of the foam material and pyrotechnics, and the concert promoters. Through attorneys, club owners said they did not give permission to the band to use pyrotechnics. Band members claimed they had permission....

On December 9, 2003, brothers Jeffrey A. and Michael A. Derderian, the two owners of The Station nightclub, and Daniel M. Biechele, Great White's former road manager, were charged with 200 counts of involuntary manslaughter — two per death, because they were indicted under two separate theories of the crime: criminal-negligence manslaughter (resulting from a legal act in which the accused ignores the risks to others and someone is killed) and misdemeanor manslaughter (resulting from a petty crime that causes a death)....

The first criminal trial was against Great White's tour manager at the time, Daniel Michel Biechele, 29, from Orlando, Florida. This trial was scheduled to start May 1, 2006, but Biechele, against his lawyers' advice, pled guilty to 100 counts of involuntary manslaughter on February 7, 2006, in what he said was an effort to "bring peace, I want this to be over with."... Superior Court Judge Francis J. Darigan sentenced Biechele to 15 years in prison, with four to serve and 11 years suspended, plus three years' probation, for his role in the fire....

Following Biechele's trial, the Station's owners, Michael and Jeffrey Derderian, were scheduled to receive separate trials. However, on September 21, 2006, Superior Court Judge Francis J. Darigan announced that the brothers had changed their pleas from "not guilty" to "no contest," thereby avoiding a trial. Michael Derderian received 15 years in prison, with four to serve and 11 years suspended, plus three years' probation — the same sentence as Biechele. Jeffrey Derderian received a 10-year suspended sentence, three years' probation, and 500 hours of community service.

In a letter to the victims' families, Judge Darigan said that a trial "would only serve to further traumatize and victimize not only the loved ones of the deceased and the survivors of this fire, but the general public as well." He added that the difference in the brothers' sentences reflected their respective involvement with the purchase and installation of the flammable foam.

Rhode Island Attorney General Patrick C. Lynch objected strenuously to the plea bargain, saying that both brothers should have received jail time and that Michael Derderian should have received more time than Biechele.