While it is a Christian duty to forgive (private Jesuit school), there is a collective civil duty to protect the public by punishing such actions.  There would have been no difference in the damage done to the 4 girls had the assailant been totally sane.  
     
One could reasonably argue that anybody who throws acid at a stranger, or even do deliberate damage to them makes one mentally ill to at least a certain extent.  Prudence dictates that civil punishment, guided by the principles of providing both short term and long term public protection must occur if any society is to long endure.