If violent crime really is more common today than in the past; and if we want to try to blame a particular technological advance for it; then the relevant technology almost certainly isn't the development of firearms, the use of which in crime hasn't changed materially since the 1830s. Far more plausible a culprit is the development of battlefield medicine and its application to civil trauma treatment. Criminals traditionally attack victims they're confident can't fight back or victims they have outnumbered, making the difference between a five-shot revolver and a 33-shot semiauto fairly academic*. On the other hand, a 19th century criminal knew there was a pretty good chance a wound from an anemic .22 pocket gun would get infected and kill him, while a modern criminal knows that if he's shot in self defense, the police will do everything they can to keep him alive until the paramedics can load him into an ambulance full of trauma gear and race him to an emergency room where, if the damage is bad enough, he'll be flown by helicopter to be worked on by the best trauma surgeon available, all while getting pumped full of antibiotics.
If gun control advocates were serious about reducing violent crime and murder rates, we'd have Mayors Against Emergency Rooms. ;)
[* - Excluding mass shootings, which are traumatic to the community but are anamalous enough not to affect the actual murder rates.]