1.

Aristocrats and gangsters have a lot in common.
They're both selfish, get bored easily, and have access to wads of cash they didn't work honestly to get and neither have any interest in bourgeois rules or morality.
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So when Thomas Shelby manages to marry into that world, to a woman whose blood is almost as blue as that cursed sapphire of his, a perfect match is the only result one could expect, a stunning result for success.

Well...

...on the outside, at least.