Originally posted by UK Vercetti.:
Meaning he was not smart enough to realise vito and michael cannot be fooled so, it was NOT the smart move. Tessio was always smarter than clemenza both in the novel and the film but his move was NOT smart he was outsmarted by the Corleone's and paid with his life 
You're point it logical and very well taken (or at least by me), however, the point that I feel that you are missing is that giving all the facts known to him,(sick Don, Corleones leaving New York, inepted successor, unable to start his family), Tessio did made the most logical decision in order to secure a properious future for himself.
Also, because a person lost in the end, it does not mean that their plan is a failure. Tessio was smart but Vito was smarter (whoever comes to Michael with Barzini meeting is the traitor).
