A huge amount of energy and time is spent lying about evil. When people start to live evil lives, they don't just claim they love evil - they lie about it, and claim it is good. And a huge amount of HOW they do that is by hiding in the grey areas and shadows of their environment, their culture, their language, all of the things they hold in common with other natives.
When they go to another country, that infrastructure is gone. They don't know the lay of the land yet, so to speak. They can't blend in cuturally, because they don't really know the nuances of the culture yet, where the grey areas are that they can hide in.
So they just act as they really are. The new culture, the new country, doesn't change them - they are merely exposed for what they already turned themselves into. Think of a predatory animal with a camoflauged winter coat going to a summer climate, before they can shed their coat. They're still a predator.
I always think of a line from the novel The Jewel and the Crown in which a middle class Englishwoman living in India berates a working class Englishwoman by saying: “India has been very bad for you.” Out of England, the working class woman had forgotten her place.
And I think America - here I’m thinking of the British father of this Oregon killer - was very bad for him. And Rome was very bad for this evil priest.