I'm going to change my answer now that I know where this is from. It's not a warning against seeking revenge, because Montresor was basically explaining how to take revenge in the proper way. So, instead, it's a guide on how to seek revenge. The meaning doesn't change much, but now I understand it to mean that the avenger (Montresor in this story) cannot ENJOY the revenge if he loses himself in anger. To properly get revenge on Fortunado, he needs to plan carefully rather than simply killing him outright. Context is very important to the meaning of a statement. Montresor wants to enjoy his revenge, and wants it to be painful and severe, so he does not want to lose focus.

This is really hard to explain, and it's not written very clearly, so don't feel bad. Basically, from what I understand, the person who wants revenge (the redresser), becomes so focused on hurting the person who wronged them, that they are no longer getting revenge, they are just trying to hurt the person. It's a warning against seeking revenge, I think.