Wickedness overall does no harm to the universe. Individual wickedness does no harm to the recipient: it is only harmful to the perpetrator, and he has the option to be rid of it just as soon as he himself decides.
To my determining will my neighbour's will is as indifferent as his breath and his body. Sure, we are born above all for the sake of each other: nevertheless the directing mind of each of us has its own sovereignty. Otherwise my neighbour's wickedness would be my own harm: and this was not god's intention, to leave my misfortune up to another.
The first part sounds like something out of a Star Wars movie as if Marcus were talking about the Force. I'm not sure I entirely understand what Marcus is saying here, but to me it seems like he's saying that just as air is all around you and available to be taken in, so too is reason and wisdom, if you but make the effort to look for and acquire it.
I assume Marcus means moral wickedness when he speaks of wickedness. When someone lies, cheats, steals or does any injustice to another person, indeed, they are only harming themselves.
Marcus further clarifies that each person's will (I like to think of it as attitude) is independent of everything, including other people's wickedness. Forgiveness is meant for you (the person who has been "harmed"). Forgiveness is the gateway to healing your soul so that it can get to the point of truly being sovereign. I would even go so far as to say that when you realize only the wicked are harmed by their wickedness, you don't even feel the need to forgive, rather you feel compassion for them and you try to think of a way to help them see their error. I'll never forget the story of Victoria Ruvolo, who was the by-standard who was hit, in the face, by a frozen turkey thrown by a teenager. She found forgiveness and more. She understands the concept of wickedness only hurting those who perpetuate it.
(see also Citadel p. 113, 270)