What causes people to betray the trust that has been placed in them?

There could be three reasons.

The first is excessive ambition, greed, lust or passion. When a person cannot control is overcome with these vices, he’s liable to betray.

It is greater than any sense of loyalty, integrity or honesty he may have.

The second reason could be a feeling that betrayal is necessary to achieve a greater good. Betrayal in this instance is not considered evil but a holy act.

A person may feel that in order to save another person's life/soul it’s ok to betray him.

The third reason could be because people like to prove how smart they are. Many people like to play with others’ minds, manipulate lives just to stir up trouble.