Montresor believes that a person should take revenge in a calculated way.
A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong. (enotes e-text p. 4)
In order to get “redress” (translation: payback), one needs to take time and make sure that the person who is in the wrong knows that he is taking revenge. Montresor plans to punish Fortunado "with impunity" (in other words, without risk of punishment). Montressor identifies Fortunado’s weak point (wine) and uses that to get him.
Monresor claims that he waited patiently for revenge. Montesor believes that revenge best when the victim does not suspect it. He is willing to wait, to take his time, to make sure his victim is off guard. As he points out, he does not make threats. He waits.
You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged; this was a point definitely, settled—but the very definitiveness with which it was resolved precluded the idea of risk. (enotes e-text p. 4)
Basically, he is saying that he took the time to make a good plan, and therefore there was no risk in carrying it out.