Putting him in charge is like putting a fox in the henhouse.
fox in the henhouse
idiomSomeone untrustworthy placed in a position of trust in which they will be harmful, damaging or detrimental for their own personal gain.
Origin
disputed originA metaphor referring to the fox as a predator that would prey on the chickens if given an opportunity to do so.
The company hired a consultant known for closing factories, which felt like putting a fox in the henhouse.
By appointing the corrupt official to oversee the ethics committee, the government effectively placed a fox in the henhouse.