There is a difference between killing and murder. Murder is the unlawful killing of a human being. There are killings which are not unlawful. Such as self-defense, or killing an armed soldier in war, or executions. This of course is a legal definition of murder and not a moral one.
If someone threatened my family then I would defend us in whatever way was necessary. If that means killing, then so be it.
The guy who killed those people in Virginia Tech was mentally ill. He was not acting in a rational manner as to whether his crime was justified or not. He was not killing people out of some moral or religious or political reason, which would have made him a terrorist.
Theft is something that is unlawful, but it it not always immoral. If I had been in Nawlins after Katrina I would have stolen food or water or batteries or other life-saving things. But I would not have stolen TVs or designer clothes or stereos. Also, there are shades of gray. If you only need 2 sugars for your coffee at McD's, and you put a handful in your pocket, is that stealing?
This is an interesting topic.