The original British Royal art collection was confiscated in 1649 by Cromwell and it's putative owner was put to death! Almost all of it was sold off, with some parts later returned after the restoration and then glorious revolution (which was more or less a coup). The subsequent collection was built up significantly by the Hanoverian's.
I expect that the crime being referred to was the execution of Charles 1st?
In the documentary they were examining a famous piece from the collection depicting Ceasar's conquest of Gaul (France), which, according to the documentary, was effectively a genocide.
I expect that the crime being referred to was the execution of Charles 1st?