I am just thinking aloud here, but I am wondering whether this horrendous attack is connected with the policy (in India) of many families to only accept male children. I have read that in parts of India only 80 girls are born for every 100 boys. This has led to a huge demographic imbalance.
Many young men - especially the poorer - have no chance of ever having a family - let alone being able to form normal sexual relationships in their youth.
In this case, the victim was a student. To some young men, she could have appeared over priveleged. She was a student, had a boyfriend and had just enjoyed a night out at the cinema. I am sure this assault had more to do with anger than any desire for sexual gratification. Of course, this is speculation - and it goes no way towards trying to excuse a heinous crime. Beastly as it was, there was a reason for this attack. What was it that turned her assailants into something full enough of rage to do this?