"Iam not bound to that all slaves are free to".Can you help me understand the meaning of this? This is a phrase Iago says when he is addressing Othello. Thanks! :)

To lure Othello into jealousy, Iago intimates he has certain thoughts about Desdemona and Cassio, but doesn't say what those thoughts are. Othello demands to know what Iago is thinking, and Iago responds, "Good my lord, pardon me: / Though I am bound to every act of duty, / I am not bound to that all slaves are free to" (3.3.133-135). Iago's point is that even slaves don't have to give an account of what they are thinking. He goes on to say that he could be mistaken in his thoughts, and that if he spoke them, he might ruin someone's reputation, because,

Who steals my purse steals trash; 'tis something, nothing;
'Twas mine, 'tis his, and has been slave to thousands:
But he that filches from me my good name
Robs me of that which not enriches him
And makes me poor indeed (3.3.157-161)

Near the end of the same scene, when Othello has fallen into a rage of jealousy, he exclaims about Cassio, O, that the slave had forty thousand lives! / One is too poor, too weak for my revenge" (3.3.442-443). [Scene Summary]
Othello Navigator: Themes: Slaves