The events from the 20th century that affect our view of absolute power today are the events that happened when certain leaders had absolute power or something close to it. We look at those events and use them to evaluate the results of absolute power.
In the 20th century, there have been a few instances in which nearly-absolute power seems to have been beneficial to a society. Perhaps the clearest example of such a thing is Singapore under Lee Kuan Yew. Under his rule, Singapore was able to become prosperous and stable in a very short time.
However, there have also been instances of horrific abuses committed by rulers who had absolute power. The most obvious of these examples are those of Nazi Germany and of the Soviet Union under Stalin. In those times and places, the rulers had near-absolute power. They used it in ways that were terribly destructive both in terms of economics and in terms of human lives.
These sorts of events condition the ways in which we perceive absolute power today. They tend to make us very suspicious of the idea of giving any person anything close to absolute power.