Some people are of the opinion that, hanging people is not a solution. Also, people ask questions such as, "Who are we to take out lives, when we cannot give life" (When vegetarians ask the same question about killing animals for food, people don't accept that)!
So, is hanging of terrorists wrong?
People like Kasab are already convinced that "killing the infidels will reserve his place in heaven"!
So, they are motivated people and you cannot make them think otherwise.
Either you have to keep them in jails and spend crores of rupees or hang them.
Keeping them in jail not only is a drain on public money, it also sends a strong signal to other
such terrorists that they can live happily even after killing people!!
You can read more about "Islamic concept of Heaven" here:
One paragraph from the above link sums it all:
"In my childhood I asked our Huzur (Imam of the Mosque) what exactly will be the likes of heavenly rewards from Allah for which all Muslims are fawning Allah by five times prayers per day, and fasting for one month. The Imam shaheb replied: “after your death you will be resurrected again as an healthy young man of 25 year old. There in the heaven you will be given beautiful Houris/Gillman, wine, and plenty of foods to eat. You will stay young for ever and death, disease will never come near you again.” Folks, all those Islamic Mullahs, Maoulanas, all those Jihadis of Kashmir, Palestine, Osama-Bin-Laden’s terrorist gang, and all other gullible Muslims ardently believe just that."
Here are some more links talking about heaven, 72 virgins, different layers of heaven, etc: