Ben's attitude

Ben is supposedly a young man of rather higher than average intelligence, yet it is difficult to accept his reactions upon finding the dead body.
He runs away panicked, then he thinks better and returns, breaking in. To remove everything indicating his presence there, probably, having decided to vanish afterwards, not reporting the murder to the police. The first action of even a little child who has watched some TV, would be to clean his fingertips out of everything he thinks he has touched.
And what he does? He puts in his pocket the knife (why on earth?) and leaves!

I find all this extremely inplausible.