Should Cameron have apologised for the Amritsar massacre?

The British prime minister became the first to pay his respects to victims of one the bloodiest massacres of Britain's colonial history - the shooting of hundreds of protesters in Amritsar, India in 1919. Should colonial powers apologise for their past mistakes? Is there an element of opportunism or convenience in a late apology like this as some commentators are saying? We get reaction from around the world.