'I'm referring to those who completely took over other people's countries - their governments, their lands, their properties; beat the crap out of the indigenous peoples' That'd be the Vikings and then the Normans in 1066?
I think that when the Brits left somewhere, they left a place in fairly good order with perhaps a justice system, or a measure of prosperity (Rhodesia springs to mind). You might like to compare that the mess that certain countries like Belgium made of places like the Congo, which has ongoing repercussions today. From what I can see many of the former Brit colonies have a fairly good relationship with us, and America seems to have survived any damage we did to it.
My problem with immigration is that the infrastructure is not in place to deal with it. There are problems for schools, doctors, dentists, housing, local councils etc. The other problem is assimilation. The councils bend over backwards to translate information into several different languages. Contrast this with Belgium. Where I live, I have to speak Flemish when I go to the Gemeente for anything; my ID card, my compost bin, my certificate to say I can work with kids; my tax return, my car tax documents; everything is in Flemish. The staff are not supposed to speak anything but Flemish to you. If you start in Flemish and lapse into English, then some will speak English to you, but many won't. The view here is that if you immigrate into Belgium you are very welcome, but you MUST learn the language, as few concessions will be made if you can't.
We also need to be examining closely what benefits the immigrant will bring to the UK. If they are going to benefit the economy or bring a needed skill, then fine. If they are benefit tourists, then no, they shouldn't come. Again, in Belgium, one has to prove that one member of the family has a job that is sufficient to support the others. You only have to look at the figures published the other day to see that already the Housing Benefit bill is larger than the combined defence and education budgets. That is unsustainable. Are you prepared to pay higher taxes to support immigration? Are you happy that in about 20 years time the population of the UK could be 70 million? That's too many for a small island methinks.