Whenever you are offended at someone's lack of shame, you should immediately ask yourself: 'So is it possible for there to be no shameless people in the world?' It is not possible. Do not then ask for the impossible. This person is just one of the shameless inevitably existing in the world. Have the same thought ready for the rogue, the traitor, every sort of offender. The recognition that this class of people must necessarily exist will immediately make you kinder to them as individuals.
Another useful thought of direct application is the particular virtue nature has given us to counter a particular wrong. Gentleness is given as the antidote to cruelty, and other qualities to meet other offences. In general, you can always re-educate one who has lost his way: and anyone who does wrong has missed his proper aim and gone astray. And what harm have you suffered? You will find that none of these who excite your anger has done anything capable of affecting your mind for the worse: and it is only in your mind that damage or harm can be done to you - they have no other existence.
Anyway, where is the harm or surprise in the ignorant behaving as the ignorant do? Think about it. Should you not rather blame yourself, for not anticipating that this man would make this error? Your reason gave you the resource to reckon this mistake likely from this man, yet you forgot and are now surprised that he went wrong.
Above all, when you complain of disloyalty or ingratitude, turn inwards on yourself. The fault is clearly your own, if you trusted that a man of that character would keep his trust, or if you conferred a favour without making it an end in itself, your very action its own and complete reward. What more do you want, man, from a kind act? Is it not enough that you have done something consonant with your own nature - do you now put a price on it? As if the eye demanded a return for seeing, or the feet for walking. Just as these were made for a particular purpose, and fulfil their proper nature by acting in accordance with their own constitution, so man was made to do good: and whenever he does something good or otherwise contributory to the common interest, he has done what he was designed for, and inherits his own.
What should your reaction be to shameless people, to rogues (dishonest or unprincipled people), traitors and people with little to no virtues? Marcus says, you should ask yourself a question. Is it possible for these type of people not to exist in the world? The answer is: no. In other words, there will always be people in the world with little to no virtue. And this doesn't mean those people are condemned to live a virtue-less life, rather perhaps, they don't know better and have yet to learn. Again - give people the benefit of the doubt; all our journeys are different.
This leads to the next point Marcus makes with regard to dealing with people who lack virtue. Marcus suggests being gentle and attempting to educate people who may lack virtue. There is no harm done to you, if you are a prokopton, for you have used the opportunity of a virtue-less person to exercise the virtues of patience and gentleness. Which leads to Marcus' next thought.
Be prepared to encounter ignorant people - expect it. And if you find yourself annoyed or shocked by ignorant people, then blame yourself for not anticipating that.
And lastly, some parting advice from Marcus - if you trust a man who is untrustworthy, clearly you should blame yourself. However, let me add a wrinkle and some food for thought to this idea. How can an untrustworthy person gain or regain your trust? Should you give them an opportunity to establish or regain your trust? To which I would respond - yes, but start little. Entrust them with little things to establish a track record and then move on to bigger things if they succeed in keeping that trust. I think age of the person ought to be considered. For a young child, teenager or young adult, we must do this. But for an ignorant person who has a long, bad track record, proceed with caution!
Also, in the last part of chapter 42 of Book 9, Marcus clearly counsels that good, right action is your duty and there should never be expectations of reward. His analogy of an eye demanding payment for simply seeing (doing its duty). Therefore, do good and leave it be. You have done your duty - it is enough.
(see also Citadel p. 201, 225-226, 271)