To go off the rails means to begin to behave in a destructive or damaging way that is unacceptable to others, such as breaking the law or engaging in anti-social behaviour. This is a casual idiom used in informal situations.

1. I think he'll go off the rails if he doesn't get what he wants.
2. I don't know why she went off the rails and hurt those people. She was always such a quiet woman.

1. What is wrong with you, you have made such bad decisions recently? Since you won the award you have gone completely off the rails.
2. The entertainment industry can be pretty wild. There are such big egos lots of parties and drugs it's easy to go off the rails if you're not careful.