His plus/minus stat is more indicative of this porous Leafs team than it is of Kessel; ... You can't pick on Kessel's plus/minus alone without considering the Leafs' overall weak defensive play and poor goaltending.
True, +/- is also very much a team statistic. Nothing in hockey happens in isolation. That being said, Kessel is part of the reason for the lousy +/- because he is weak defensively. He probably costs the team almost as many goals as he scores.
You need to surround a player like Kessel with strong defensive players that can pick up the slack. The Leafs cannot do this.
But I digress..... I have already mentioned the reasons why it would be smart to trade Kessel at this point. It is buried somewhere in this thread.