I went to a small high school - about 600 students. In my senior year, there were 2 suicides - one of whom was a friend of mine. There had never been a suicide at that school before.
In the last 2 years at my workplace, 3 of my co-workers (from the same department, of about 9 people) have had divorces or separations.
In 1954, for the first time in recorded human history, Roger Bannister ran one mile in less than four minutes. Two months later, he did it again, as did his competitor, John Landry. It's immortalized in this statue at the fairgrounds in Vancouver where it happened.
What's with all these things happening in clusters? It's like watching an ice mountain thaw and crash into the ocean (c'mon, you all watched An Inconvenient Truth, didn't you? Those poor polar bears...). One small action is the trigger for the whole avalanche that follows.
So, when the precedent is that someone trains hard and breaks the invisible ceiling on running a mile, that's great. But when the precedent is that the first plain old Hutu civilian picks up a blunt object and beats his Tutsi neighbour to death, it's a whole different story (too extreme? well... it's true. Deal with it).
Precedent is an empowering thing. In itself, it isn't good or bad. But it can be a virus that infects people's brains. The fact that someone 'did that', compels us to want to try it too. Like a kid who wants a toy just because another kid (usually MY kid) is playing with it. It can motivate or free us to do things we otherwise wouldn't have imagined doing. Or it can make us want to do things we never really wanted to do.
It doesn't hurt to be aware of the power of the precedent. "If she can do it, so can I" is a very different statement than, "If she can do it, so SHOULD I".