Personally I have no problem with India's decision to execute this man, who gunned down many innocents at Mumbai's railway station, nearly four years ago.

The BBC's local reporter, in a good report, noted that:
many believe, the government, beset by allegations of scandal and inaction, timed the execution perfectly.

It came a day before the beginning of the winter session of the parliament, where it's primed to face a noisy and tough opposition. It also came five days before the fourth anniversary of the attacks when the main opposition BJP would have almost certainly reminded the government of its "failure" to execute Qasab.
Link:http://www.bbc.co.uk/news/world-asia-india-20422270

Some of the victim's reaction are in another BBC article; this says it all:
...the sense that someone has been finally held accountable for the deaths makes the sense of injustice slightly more bearable than before.
Link:http://www.bbc.co.uk/news/world-asia-india-20423470

A comment on the Indian Twitter surge:http://www.bbc.co.uk/news/world-asia-india-20426516 and the Indian press reaction:http://www.bbc.co.uk/news/world-asia-india-20440455