Commentary on Meditations: B9:1

Injustice is sin. When universal Nature has constituted rational creatures for the sake of each other - to benefit one another as deserved, but never to harm - anyone contravening her will is clearly guilty of sin against the oldest of the gods: because universal Nature is the nature of ultimate reality, to which all present existence is related. 

Lying, too, is a sin against the same goddess: her name is Truth, and she is the original cause of all that is true. The conscious liar sins to the extent that his deceit causes injustice: the unconscious liar to the extent that he is out of tune with the nature of the Whole and out of order with the nature of the ordered universe against which he fights. And it is fighting when he allows himself to be carried in opposition to the truth. He has received the prompts from nature: by ignoring them he is now incapable of distinguishing false from true.

Moreover, the pursuit of pleasure as a good and the avoidance of pain as an evil constitutes sin. Someone like that must inevitably and frequently blame universal Nature for unfair distribution as between bad men and good, since bad men are often deep in pleasures and the possessions which make for pleasure, while the good often meet with pain and the circumstances which cause pain. 

Further, anyone who fears pain will also at times be afraid of some future event in the world, and that is immediate sin. And a man who pursues pleasure will not hold back from injustice - an obvious sin. Those who wish to follow Nature and share her mind must themselves be indifferent to those pairs of opposites to which universal Nature is indifferent - she would not create these opposites if she were not indifferent either way. So anyone who is not himself indifferent to pain and pleasure, death and life, fame and obscurity - things which universal Nature treats indifferently - is clearly committing a sin.

By 'universal Nature treating these things indifferently' I mean that they happen impartially by cause and effect to all that comes into being and owes its being to the fulfillment of an original impulse of Providence. Under this impulse Providence set out from a first premise to establish the present order of the universe: she had conceived certain principles of what was to be, and determined generative powers to create substances, transformations, and successive regeneration.

In this lead-off passage from Book 9, Marcus uses a lot of words to describe various sins.  In the first part, the sin of injustice is committed when rational creatures (humans) harm each other.  And he further sins when his will is not aligned with universal Nature.

In the second part, he calls out lying and describes how intentional liars create injustice as well as unintentional liars, who ignore the prompts of nature, which are trying to show him Truth.

In the third and fourth parts, he calls out those who seek pleasure and avoid pain in their pursuit of contentment.  These people will in-justly blame universal Nature for picking favorites.  Misfortune falls on all; and universal Nature does not bestow pleasure on just the good.  Everyone must realize that pleasure will come and it will go - the same with pain.  Seeking a life of virtue truly is the sole good.

Lastly, Marcus reminds us that universal Nature sees pleasure and pain indifferently and we too should view them the same.

(see also Citadel p. 213, 270)