There is a better alternative.
The President of India has the power to pardon death sentences in exceptional cases. Similarly the President should have the power to sentence criminals to death in exceptional cases. However in the current political scenario in India, to prevent misuse of this power, such a decision must taken by the President without the advice of the Prime Minister.
In the context of Kasab's case,in my opinion the President definitely has the moral authority. Afterall the President is the Commander in Chief of the armed forces of India and when someone has waged war on India, then the Commander in Chief should have complete legal authority to consider this case just as if it were an enemy soldier being killed in war. It should be the precedent for all such terrorists who are caught in the act. For those arrested on information, prior to action, suspicion etc. the regular legal process must follow.