Why are kids so cruel especially when it comes to bullying?

Some kids are cruel, some are observers, some are kind.

The average kid focuses on ‘belonging’ and ‘fitting in.’ Enlisting in any group carries with it requirements that you prove solidarity. Disagreeing with group activities is a good way to get kicked out of the group. Belonging to the RIGHT groups is an important choice to make.

Otherwise kids find themselves endorsing things they would not engage in individually - like bullying.

Let's start at home and see what's going on there? How does the child spend his/her free time?

Instinctive predatory dominance.

Empathy and sympathy are higher level reasoning that their brains have not developed for it yet.

What everyone wrote, there are many reasons.

The learn it their parents

The strong subjugate the weak

Blame the parents and teachers