I think this isn't about ethics at all and I wonder why you think it is. It is about size, economics and power of the UK empire in the 1800s. Not being (as much of) a cruel tyrant tends to help with all of these, though.
Also the article starts with an implicit assumption of survivorship/observational bias, the title being 'Why were Victorian elites so effective?' - the assumption that they really were 'effective' or was the rest of the world simply worse and they were simply the 'least worst'? That line of enquiry was not even discussed.