Cruel King
Who was the most merciless and cruel king in the world?
Mihirkul was most cruel king. He loved cruelty. He would make elephants fall down form high hills and would rejoice in cries of agony of the poor elephants.
Nadir Shah was another cruel king who massacred many in Delhi.
Mahraja Ranjit singh is said to have terrorized afghans so much with his cruelty that even now Afghan mothers silence their babies by saying that Hari singh Nalwa (Ranjit Singh's general) would come.
There are many others but I have referred to kings only.
@Captain2010