In chapter 999, Big Mom still showed concern about Kaidou(Onigashima) landing on the Flower Capital , which would result in thousands of subordinates being killed, while Kaidou didn't seem to care and said that new slaves are easy to obtain. This is just one of the countless other insane atrocities committed by Kaidou.
Even under the regime of Orochi, which was falsely led by him, it wouldn't be as bad if Kaidou didn't exist. There wouldn't be polluted rivers, and even the number of slaves in Udon would be smaller, orochi wouldn't even be alive if Kaidou hadn't saved him when Yasui challenged him.
With Kaidou in power, there was slavery and famine, and women were killing their own babies. Kaidou went even further with the new Onigashima, where even children would be enslaved, he called Orochi, who was already nothing decent, a weakling.
An admiral fearing Kaidou is a result of numerous stories about his brutality. Since Big Mom doesn't have this level of presence in the world of One Piece, it's obvious that Kaidou is winning in this regard.