If it's true only one person was killed in the initial shooting, they probably thought it was a relatively simple, I-hate-my-girlfriend thing and didn't see any sense in locking down the whole campus. Obviously that turns out to have been a colossal mistake.
However, it's worth pointing out that there's no telling how much of what we're hearing now is going to turn out to be accurate. As I recall, originally they thought twice as many people had been killed at Columbine as it turned out to be. It'll probably be tomorrow before we start getting much accurate information about what went on today.