How can power make men cruel to his fellow man?
The question of absolute power and its abuse is not one that can be answered simply. Instead, one needs to analyse the various components and assumptions underlying the question.
First, there is the matter of a "post hoc ergo propter hoc" fallacy. Does power make people cruel? Or is it that cruel people, when in subordinate positions, have less scope for cruel actions than when they become powerful?
Next, there is a sociological issue, or issue of observer bias. Cruelty in the rich, famous, and powerful is more likely to be noticed than cruelty among the poor or marginalized. A young child pulling the wings off flies is cruel, and so is a dictator killing thousands of people, but the news is not filled with stories of petty cruelties by average people.
Next, are cruel and ruthless people more likely to gain power than nice people or does power itself corrupt? Lord Acton, a distinguished historian, was particularly concerned with these issues -- see below for references to his work.