On not becoming the argument
“The best revenge is to be unlike him who performed the injury.”
This is not forgiveness as warmth. Marcus keeps the word revenge and simply redirects it: the retaliation he recommends is remaining someone the other person is not.
The difficulty is that injury teaches by imitation. Being treated coldly is the fastest way to learn coldness, and the learning happens whether or not you approve of it. The line asks you to notice that transmission and decline it, which is harder than getting even and much quieter.