@Catchabula,
First I would like to say that there is no reason to fell sad for those who died, because they no longer suffer, but rather one should fell sad for those who didnt died or for the people related for those who did. It helps to think this way because the people who stayed are also those who can be helped.
The answer is probability I believe, that is, althought the probability may be small, there is always a chance a certain event will happen, such as one person deciding it has to kill children. It helps to understand it by seeing that it is just as an randow accident that could have happened, for example, a sizable meteor could have hit the creche and annihilated it. If meteors can fall and kill people, then people can be born with weird mental diseases wich eventually cause then to do things like that. Its just hard to understand because both such level of blood-coldness and such level of hatred against humans are rare, even more rare combined.
I think the only way to deal with this is to learn to control emotions, that is, harden your mind against suffering from the bad events that happened outside of your control, and open it to the good ones.
And one thing to think: And inteligent and radicalist friend of mine said the main reason this kind of thing keeps happening is because some people get tired of life and decide to "leave it with a bang", and they would do so knowing that everone would know about what they did, and that they would at max be imprisioned forever or executed painlessly. He said that the solution would be that people who did such things were tortured to dead in public. He gave as example one country (I dont remember the name) where if someone kills someone, the closest relative of the victim gets the right to shot the killer 20 times, being the only condition that the last shot must be on the head. The country has one of the lowest homicide ratings in the world.