DIG ONE’S OWN GRAVE

Meaning: when you dig your own grave, you make an action that results bad consequences for you in future.

For example:

Everybody, who eats in McDonald’s 5 days a week and do no exercises they’re digging their own grave.
In our mafia Family we have an unspoken tradition: if somebody comes late to our closed meeting we kill that person for the 1st night in an upcoming game. So you dig your own grave on a constant basis by coming late all the time.

You can practice this idiom soon at one of the English Mafia Club’s upcoming meetings: https://englishmafiaclub.com/events/upcoming-events