Adam Clarke on Jeremiah 40:16

Thou shalt not do this thing - He cannot be so base.

Thou speakest falsely of Ishmael - He thought it quite possible that the man who was capable of becoming an assassin was capable of telling a lie; and therefore he would not credit what he said. Had he been a little more distrustful, he would have saved his own life. The next chapter shows that Johanan's information was too true. So noble Gedaliah lost his life by not believing that evil of others of which he himself was incapable.