dig one's own grave
Seriously harm oneself, cause one's own ruin or downfall. For example, If Sam pursues that course, he'll be digging his own grave. [First half of 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.