I agree for abolishing death penalty for Indian citizens. But what about foreigners?
Kasab was not convicted of manslaughter (killing someone in a car accident for example) or even a crime of passion like culpable homicide (killing someone who robbed your money). There are justification for not handing out the death penalty for manslaughter or culpable homicide. There might even be justification for not handing out death penalty for pre-mediated murder not committed out of passion (like murder for example). But what about people like Kasab who was convicted of waging war against the state? I believe that this is the reason why he had trouble getting clemency or overturning his sentence on appeal.
Now you could argue that there should be no death penalty under all circumstances -- including waging war against the state. If so, how do you then justify wars? Shouldnt our army first go and arrest combatants, try them for waging war against the state and then lock them up for life? The reason they shoot them at sight is because of the legal justification of catching someone red-handed while waging war against India.
Yes you can argue against death penalty under the following heads:
1. Immorality. Wars are immoral too. So let us disband our army. More people have been killed by IA than death penalty handed out.
2. Irreversibility. Jailing and time spent in jail is irreversible.
3. Deterrence effect is not proven. Neither is the deterrence effect of war. We have had war with Pakis in 48, 65, 71, Kargil and low intensity conflict. War with Chinese in 62. Do either the Pakis or Chinese given up on trying to attack us? Should we not renounce war and arrest people who attack India instead?
You can question if Kasab was inherently bad or just a brainwashed pawn who could have been redeemed after a few years of jailing. But that is like trying to argue that everyone one of us are made of protons and electrons and there is no inherently bad proton

. If we accept that he committed the crime of waging war against the state, which has been proven beyond doubt, while treating him in a reasonable way, giving him proper legal representation, giving him right of appeal and right to clemency, then we should as a civilization, execute people who wage war against our state.
You can now argue, why do we want to execute people who wage war against our state? That is akin to asking "Why do we want to survive as a country?" I mean surely you cannot argue that Delhi has some kind of divine right to rule over all parts of India and follow some made up rulebook called the constitution? I mean, India is not the greatest country in the world so why do we have the right to protect our survival and kill those who attack us?