as that will become a usual thing which may result in more violence.
They believe that nobody will do the same to them. And that is why they're acting this way. 
Humanity should be turned off while you're making any judgements related to their activity, because they're not humans. Animals don't deserve your tolerance. Their execution should be cruel enough to make them think a bit, pure logic and no amotions here.
An eye for an eye will make the whole world will blind. It isn't the solution my friend. The only solution, we should make them not happening again. As I see, it can be only done by education.