Please imagine you trust someone who you don't know them well and e.g. ask them to deliver a considerable amount of money to your friend somewhere. Prior to sending it, you made a call your to friend and had coordinated with him that someone will bring you the money by a couple of hours. Meanwhile, according to your phone conversation your friend has been supposed to make a call to you after taking delivery of the package containing that amount of money.
You send it by the guy and several hours passes and you doubt if there is something wrong here. You call your friend, and he says there had not been any delivery person at all. You call someone who knows the guy and tell him the story.
He wants to say you have chosen a bad person to do such a thing and you had not to trust him at all.
We have a very popular proverb here which says: "you have given the meat to the cat" meaning that the cat will never overlook a piece of meat and undoubtedly will eat it.
The only proverb I found is:
- You have set the goat to watch the cabbage.
But I doubt if it sounds natural in this sense in AmE. If it sounds weird to American's ears, then please let me know what you would say instead.