> hang them with their own petard
You can't hang someone with a petard, but you can be hoist with your own petard.
"Hoist with his own petard" is a phrase from William Shakespeare's play Hamlet that has entered proverbial use in English. Literally, the phrase means a bomb-maker was blown off the ground (hoist) by his own bomb (petard). It commonly refers to an ironic reversal in which one is taken down by one's own scheme.