Now, don't misunderstand. That was wrong of him. And it was also stupid. But there is a very important difference between finding out about an illegal transaction and participating in one.
If it were prostitution, it would be the difference between watching a john pick up a hooker and arresting them both, or posing as either a john or a hooker and arresting the other when they eventually committed to the crime. The former is acceptable. The latter is not.
It wasn't a case of a contract killer ratting his client out to the police. There never was a contract killer.
In my opinion, if the cops were a necessary component in the commission of the crime, it is entrapment. If they were simply able to exercise a degree of control over the situation such that they are able to immediately arrest all parties to a crime before anyone gets hurt, that's a little creepy, but still acceptable.
Crime requires intent, conduct, concurrence, and causation. Entrapment elides over one or more of those elements.
They absolutely did not. It was his idea. You need to go learn what entrapment is. Hint: It's not simply when you do something and the cops happen to be involved.