It appears to be an example of poor law enforcement, not an example of why gun control laws don't work.
I understand the point of "see look, criminals still get guns, regardless", but in this situation, it seems like the criminal never really payed his debt to society. If he were, he would probably still be in prison, not on the streets. Although I would definitely accept that is only prolonging the inevitable. Whether he came out of prison now or in a couple of years, he would still have been able to get a gun (in spite of the laws) and he still would have used it.