It's truly remarkable that this is such a rare occurrence when you think about it. Neck guards have been around a while, but they are almost non-existent at the highest levels, or at least have been. In the aftermath of it, the Penguins (those being his former team from when he briefly reached the NHL) made wearing them compulsory for their AHL and ECHL affiliates, but the NHLPA has to agree to introduce them at the NHL level.
A lot of these Euro leagues, especially the smaller ones (i.e. not the NLA, SHL, KHL or Liiga), have a mixed level due to the nature of hockey as a fringe sport in a lot of those countries (Germany, Austria and Czech Republic also have some decently strong hockey tradition but their leagues are smaller than the above) and so you've got everything from ex-NHLers like Johnson down to ham-and-egg semi-pros with vastly different skating levels on the same lineup. I've seen a lot of criticism of the league for its medical provisions in the aftermath of this - but also others saying that he'd lost so much blood he was virtually unsaveable within just a few seconds, and it is worth noting that Sheffield and Nottingham are two of the best resourced and best-run UK hockey teams; I've also seen plenty of people throwing some awful accusations towards Petgrave as a result. I feel confident it's entirely accidental; he intended to throw a hit but either caught an edge or was thrown off balance by another and made a bad decision, but either way I'd be surprised if he ever plays much hockey again just from guilt or PTSD without adding in potential abuse from the crowd or opponents.
At the end of the day, unfortunately it is a full-contact and violent sport, and also one where every competitor takes to the ice armed with razor sharp slivers of metal attached to their feet. Something like this is always the worst case scenario, but it always seems so unlikely to happen until it does.