MaizieD, I dare say the answer is, that once this super-ruthless type gets into power, they will usually keep similar types in their immediate circle, and by the time they have assumed power, other people are too afraid of questioning or disobeying them since they know it will mean execution, and/or torture, labour camps, etc.
Re Hitler at least, The Rise And Fall of The Third Reich, by William L Shirer*, is a riveting read, even for someone like me, who wouldn’t usually read this sort of thing at all.
*an American journalist who was there during Hitler’s rise to power, and during the early days of WW2, and also wrote A Berlin Diary at the time.