What is wrong with suggesting sin?

Hashem asked Adam why he ate from the forbidden fruit. He asked Chava too. He did not ask the snake, rather He turned to it and decreed punishment without further ado. Why didn’t He ask the snake if it had anything to say for itself? Rashi explains that the snake had a lot to say for itself. It would have said “So what if I suggested then to eat from the fruit – are they so stupid as to listen to me and not to You?” Therefore it was not asked, so that it could not offer an answer. Rashi says that here is an example of the principle that we do not try and look for mitigating factors when dealing with a mesis – a provocateur who tries to convince people to do bad. First off, if the crime of a mesis is convincing people to do wrong, what defense is there that they ought not to have listened – the crime is doing convincing? It seems the crime is causing sin to happen, and for that the defense is that the fellow ought not to have listened. Problem is that it just isn’t so. Classical mesis in halacha is when one tries to convince another to sin with avoda zara and the fellow snitches on him to Beis Din – i.e. no sin was actually committed. So mesis means trying to convince someone to sin. If so, pushing sin is the crime. So what defense is it that the guy ought not to have listened?