Think the product of their time argument is a bit of a cop out. The fact is that the American public had to be whipped into a frenzy by a massive propaganda campaign in order to gain support for entry into the first world war.
In 100 years will the history books talk about the Iraq war as acceptable at the time? Or will they mention the massive protests and 80 to 90% public opposition to the conflict in Britain and Spain?
And saying 'if we hadn't done it somebody else would have' is an absolutely horrendous defence. It's like seeing a jewellery store window broken and helping yourself to a few watches and gold rings because somebody else will if you don't.
Just to play devils advocate, would you support Spain paying back the riches they stole from the native South Americans, and compensating them for the loss of lands and mass genocide? What if it means Spanish people having to pay more tax?
It's simply not a case of saying 'if we hadn't done it somebody else would' because every European nation worth it salt was doing it. Britain just did it better than the rest. Although Spain and France gave them a run for their money. Even the USA were at it, they where just clever enough to call it something else. Germany simply arrived late to the party, and that's what eventually lead to the first world war.
At some point a line must be drawn under it all. It's history now. There isn't an empire that has ever been benevolent, they can't be by definition. I think us British people can be proud of what a small European island managed to do, whilst simultaneously acknowledging the bad it did to achieve it. For some reason, it's fashionable for British people to hate the empire, anyone who thinks differently is usually labelled as some sort of nationalist xenophobe.
Personally, I think we have managed de-colonisation badly. Whilst giving these nations their freedom was the right thing to do, it was badly done. And we haven't done enough to support our former colonise since. The Commonwealth being no more than a tokenistic gesture. That is the bit we should be most shameful of. We can't change what we did as an empire, but we could of given our former colonies one last gift; peace and a prosperous future. We at least owed them that much.