Cruel ways of inflicting suffering may involve violence, but violence is not necessary for an act to be cruel. For example, not helping the one who drowns and begs for a helping hand, and watching it with disinterested amusement or pleasure instead is not an act of violence, but is an instance of cruelty.
Cruelty is inherited in human nature, and some apes are capable of cruelty too. No other animal is known that feels pleasure in seeing other suffer. According to Friedrich Nietzsche, almost all higher culture comes from spiritualization of cruelty.