It was the leaders of the countries who were responsible for all the damage caused by colonialization -- not the general population. But they're not the ones who are affected by immigration since most of them live in upscale neighborhoods. It's mainly the working class who are affected and at times being replaced.
That said, European colonialization has caused enormous damage around the world, as has U.S. regime change of foreign governments, so we're all responsible for the migration crisis.
What a mess we've made of the world.
I don't agree with first sentence - everyone who participates in something is responsible - we cant just excuse things on 'following orders of leaders'
I do agree with your second two sentences