That is not a question of change within centuries, that is various people at that time making active decision to keep or abolish slavery.
Napoleon contemporaries did characterized him as power hungry tyrant. This is not just projecting current values. People in the past had opinions too.
There's a trend of assuming that people in the past were simple, without agency and that society was uniform. Contemporary people do make own decisions, some of them harmful and egoistic. We don't blame current political decisions (including stuff like what Isis did) purely on circumstances. The past people deserve the same treatment.
People who study politics absolutely use circumstances to explain political outcomes. Historians discuss the economic impacts of WW1 when talking about the rise of Hitler and Nazi Germany. People discuss the complex political history of the middle east and its long history of resisting superpowers combined with the power vacuum when the US left the region when discussing the rise of ISIS. Power vacuums and food/water scarcity are often the root cause of political instability.
Crazy individuals do crazy things, but those crazy individuals gaining power is always a result of the political system in place at the time.
The discuss it a little. The books I have read about rise of Hitler did not blamed "economic impacts of WW1" all that much. That was far from being a single or primary thing.
The choices, values and opinions of major actors plays huge relo.