I am not justifying the killings. However, within that girls community there is a strong sense of what is right and what is wrong. She effectively brought shame to her entire family........that is how they see it. So I can see why it happened.
In these cases the family is put before the individual (families can benefit greatly from arranged marriages). It could be argued that this benefits more people than get hurt by it. In the west we tend to do the reverse........individuals needs come first.