Like when someone discovers something about their government that was suppose to be hidden & they get eliminated because they found out about it.
A person who's seen too much or knows too much - that's the usual idiom.
The single word that comes closest is liability, but that's more generic and you need to put it in the right context, for instance:
"You just know too much about our operation, Mr Quinn. You've become a liability, and as much as it personally pains me, you have to be eliminated."
(That could be a cliched line from any cheesy espionage thriller).