How could they do animals cruelty to the animals?

Animal cruelty happens for an infinite amounts of reasons. The following, I believe, are the most common.

One cause is that the abuser is very angry and needs to take it out on somebody. Domestic violence can often include animals in the equation: Sometimes a pet is beaten and/or killed to spite the owner, meeting a similar fate to what happens to children in unstable families. Sometimes, workers handling livestock vent their frustrations out on the animals.

Another cause is when a lot of money can be made out of abusing an animal. This is rather frequent in slaughterhouses, fur farms, dairy and egg farms. Some owners just want to increase profit at the expense of the animals' suffering.

Another factor is pure ignorance. Not everyone understands what many animals feel, so they think that they can do whatever they want to an animal with consequence.