Because society should not condone (nor aspire to be) a murderous scumbag like Van der Sloot.
I am all for making turnabout fair play. When criminals consider that whatever they do to their victims, they too shall receive as punishment. That alone would reduce violent crimes.
This idea that you can slash somebody with a knife then you should be, no, must be treated with kid gloves, disgusts me. I am one who supports broken bones with an an equal number of broken bones.
Attack my family and I will obliterate you and yours.