There's been arguments in Finnish media that we should make sure that never again will there be a school kid who goes on to kill a number of fellow citizens. People have suggested outlawing guns in homes and all other kinds of things to accomplish this.
Those who do so, do it without the faintest understanding of statistics. A country cannot control the environment kids grow in, not to the degree it would need to in order to prevent some of them from going on killing spree. But a country (and especially parents and other people) can do well and keep the probability of that happening low.
In Finland, we had a bombing case about five years ago, now this killing. Before that I remember a killing spree from the time I was in school, probably early 90s. So it seems that with the current culture and population, the statistical occurrence is about one case per five years. Which is not bad, really, though it could be better.
The problem with trying to figure out how to get to never is that you'll never consider the ideas that might actually reduce the possibilities of this kind of thing happening. I also don't think that people who say that really think their facts through. There's approximately 50000 kids each year, which makes for 250000 kids per five years. So the occurrence is one in 250000. (Actually there are probably more kids who kill, but since they don't do it on school grounds, they don't end up in news the way this guy did).
So, instead of trying to think "never again" think "how can I make this even rarer". Otherwise you'll end up guilty of either wishful thinking (not good for making useful things happen) or quite totalitarian control (as you need to control all those 250000 kids to make sure that one doesn't do bad things), or both.