Why do people do crazy things?

I know this incident has happen awhile back ago, but it still haunts me and surprises me of the fact that how can people do such things. What happened at Virginia Tech several weeks ago is unbelievably crazy. For those that have not known about it, an international student at Vtech got decapitated by a guy she's only known for 2 weeks. The reason was never proposed, but if you think about it, what other reasons can there be? The girl has only known this guy for 2 weeks, it's not like it could've been a jealous situation. It's just really stupid that this guy who's known to be intelligent, respectful can do this Even worse, now the girl has to be brought home to her family in China, decapitated and buried innocently. It just breaks my heart. Not only is her family affected by this situation, but also the student body at Vtech. Still recovering from the traumatic massacre 2 years ago, another traumatic scene comes up again like this must be really hard to intake for these students. By reading the newspaper or just watching the news on tv, you'll see that there would always be a person killed or injured for stupid reasons. I ask myself everyday, "why do people do crazy things?" Do they not think before they act? It's amazing what goes through ones mind to be able to do such things like this. How could you be able to hold responsiblity and live with the fact that you took away innocent lives? ...:/