The likelihood of the Devil's existence?

If the devil is meant to be the "personification of evil", then why does he punish those who please him with wrong doing, is it not more sensible that they be rewarded for carrying out the devils work on earth? Its almost as if the devil is a third party that god has chosen to outsource the punishment of sinners to, rather than have their punishment attributed to himself. Is it not, in reality, more likely that the devil is merely an invention to force people to fear punishment in the next life in order to promote conformity in this? And does this by extension not take away from the credibility of gods existence? The bible gives little insight into this perplexing area.