Do u really thnk Tobi is a guy to follow pacts. He planned Itachi's death by making his encounter with Sasuke successful. Then only he could dare to send Nagato to destroy Konoha. Nd the greatest secret of Tobi would be he is nt Madara bt Obito. Itachi was a little afraid of him thnking him to be Madara. If he knew he was Obito nd that he was tricked, he could easily kill him. Nd so would Nagato. Nd supercool Itachi would surely find some way to counter him. Nd if Tobi believed Itachi could kill him, then its 100% true.