I would go with Spain.
What they did with the natives of Americas is unthinkable.
Just spotted this fascinating thread - great one Riddle.
Living as you do, in India - you really should be under no illusions here.
In the roll-call of colonial infamy; which includes such luminaries as the Spanish, the Dutch, Germany, Imperial & Soviet Russia, China, Japan, the Ottomans and lesser ones - two stand out above all others.
Yes, take a bow USA & the grand-daddy of them all , dear, lovely, cuddly little old Britain!
Just enter "deaths caused by colonialism" or something similar into a search engine and the name of the quaint little island that I live in will ring out like no other! Credible estimates range from 100 million to five times that number. Indeed a very scholarly article I have read says 1 billion. Most of those deaths occurred in the sub-continent & Africa.
There is absolutely no doubt that the Brits caused the deaths & impoverishment of many, many more people than any other nation in human history. Strangely they are also majorly more responsible for world economic advancement from the industrial revolution onwards than any other nation.
