Nat said nothing more than stabbings are bad and assumed it was a violent crime, a sane reasonable assumption given the information available.
What exactly is the problem? He genuinely believes violent crime is up and that this was some random hobo. I don't recall you or anybody else even theorizing this was a stabbing by somebody who knew Bob personally.
You think his opinion is wrong and therefore he is "biased"? And if this turned out to be what he thought you'd say what exactly in that case? Maybe you're the one who is biased.
At some point people got to stop playing this aha, gotcha games and agree on some base reality. Either violent crime is up or it is not. Some people like Nat are genuinely convinced it is based on their experience living there.