The concept of conquest does not imply "crime" as it occurs at the intersection of two societies with different laws, and in most cases neither would consider a conquest benefiting them to be a criminal/negative activity.
I have never read a definition of "land theft". If it is supposed to mean the same as "conquest", it goes about it in a confusing way, by including the word "theft" which is a crime, whereas as we saw, "conquest" cannot be a crime in the traditional sense. It's also unclear to me why use "theft" rather than "robbery" which seems more appropriate given the violent nature of conquest.
I guess the point of using "land theft" instead of "conquest" is to imply that in an ideal world there would have been a supranational law enforced on all human groups that forbids violent displacement of one group of humans by another (i.e. conquest), which I guess most people would agree would be a good thing.
So, assuming conquest and land theft are synonymous, with the latter being used to express disapproval, and going back to your first comment: "To say that it was anything but utterly disgusting is an understatement". I suppose this is true, but the reply from refurb seems appropriate: humans have engaged in warfare long before we have written record. Our cousins, the chimpanzees do abominable things to each other in war, and so did our ancestors, of every race and every culture, including the native people that were the victims of the British empire.
"What hypothetical American indigenous tribes may or may not have done is not the subject of this thread" is a silly reply, as far as I can tell. American indigenous tribes engaged in warfare and conquest [1] like every other human group in history.
The British Empire did not, as far as we can tell, come up with new and particularly hideous ways to inflict harm upon other humans, that we had not considered in the hundreds of thousands of years we've been harming each other.
They have the distinction of being the first ones to do it globally. The true harm of the British empire is the delta between the harm they caused to humans in the places they occupied and the harms other humans (the ones conquered, others in the region) would have caused in the same area over the same period of time. Unfortunately, I don't have a way to estimate what that delta is. But I am sure the answer is not "all the harm the British empire caused minus 0 because everyone they conquered was a peaceful post-warfare civilization, disturbed only by the British savages".
[1] https://www.canada.ca/en/department-national-defence/service...