I find that in the course of angry letter writing, you are walking a balancing act in which you have an action taken that could be motivated by bad faith or by incompetence or an incomplete picture of the situation, so it's best to approach the subject in a clinical manner and lay down the factual merits for your case, rather than your emotional merits. Even if you do not believe it, your case should always look as if it gives the benefit of the doubt to the intended reader.
That isn't to say you should hide your emotions, but rather state them in a neutral and factual way. Rather than accuse the reader of being a monster, explain that you are having difficulty in rationalizing in any way that supports the reader's actions. It's critical that you lay out what you did see and why the action is being perceived by you as a bad faith action. Never directly accuse.
Don't start by saying "You are a monster for doing this and if you don't fix it, you will always be a monster." Rather, use statements of facts: "I am writing to inform you that I am having a difficult time understanding your recent actions and in light of this difficulty, I am having trouble with justifying your leadership capabilities to myself."
This does a few things. It gets what you emotionally feel out, but in a factual way. It states the cause of these emotions, in this case actions that seem to violate rules, and you admit you don't know what the reasons for these rule violations are, and allows the person the space to explain his or her side of the story and justify what happened.
From this, lay out the nature of the incident in question from your perspective, the rules that you perceive that were violated by the actions, and outline what you believe would be the correct actions of discipline under the law. Try to avoid shifting to unrelated rule violations or discipline for them. Try to limit example cases of past enforcement as the circumstances may be different or the rule did not exist and post ipso facto enforcement is not allowed. Doing so, especially excessively, can come off as whataboutism, and implies that the rule is being selectively enforced based on the reader's bias. Again, the point is to outline the problem as you see it and allow them a chance to justify the actions to you. Accusing them of hypocracy when they don't see it that way is an accusation of bad faith.
Especially when challenging someone with rule enforcement power, your letter should always assume the best, but it's okay to expect the worst. It may come that the enforcement agent will target you for charges because you are accusing him of things which he did not do, especially if you don't have insight into his perspective of the situation. The longer you can keep from an emotional charge or an accusation, the better, as you are more likely to draw support on the merits of the case than you are by impugning the character of the person acting under color of law. Consider Ghanid and Martin Luther King Jr., whose passive resistance to the law made the responses to them by law enforcement less easy to defend than if they have fought back. Arresting a man shouting "Hey Hey, Ho Ho! Racism has got to go!" at a segragated diner was a lot easier to justify then arresting a man for sitting down and ordering... for the crime of having the wrong skin color to make the order in that chair. In fact, in the case of a few department stores who's dining sections had this, they ended up repealing the rule long before the Jim Crow laws that allowed it were themselves repealed. The disproportionate reaction to a rather small infraction of the law built outside support for those who said that they were a problem, as the laws were justified as a way of stopping criminals from committing crime, not from people eating lunch.
Because of this when writing angry letters, you should attack the logical weakness of the situation, "Why was it done this way when the rules say not to?" and not the emotional implications that it is because they are "Evil."
If you are going to appeal to emotions, it should be done in a way that use your factual arguments to lead to a visceral conclusion that the reader would be opposed too. If you can decouple the reader from the sitution... force him to read from a narrative perspective where he is looking at a situation from the outside and upon reaching a conclusion, comes to view his own actions as similarly problematic. Two well beloved users of this tactic were Jesus, who's parables were used to put the theory of his preaching into a simple story that others could relate too, and Star Trek, which took real world modern issues and re-imagined them as if it was aliens with the same problems. In the Parable of the Prodical Son, Jesus tells a story of a father celebrating the return of a child who left home and lived fast and hard and fell on difficult times and caused his father emotional grief, as a way of relating to his audiences the unlimited forgiveness of God, and how God would respond to even a sinner who returning after previously turning his back on God. In Star Trek TNG, the episode "Measure of a Man" is not only a good example of this, but it also "shows the trick" being pulled on the audience in the story.
The plot in question, shows one of the main crew, an android named Data, in a trial to determine if he is Starfleet property like a computer, or has rights like Starfleet crew. The opening argument consists of displays of Data's android nature, from his superior strength to the detachment of limbs, and even the location and functionality of the off switch. The argument is so devastating that Picard (acting as Data's legal council) finds himself alone, late at night, unable to mount a defense). The bartender (played by Whoopi Goldberg to add more punch to the conversation) discusses the conclusion of the ruling if it favors this argument, as Picard admits, he's nearly convinced and cannot factually attack it:
GUINAN: And now he's about to be ruled the property of Starfleet. That should increase his value.
PICARD: In what way?
GUINAN: Well, consider that in the history of many worlds there have always been disposable creatures. They do the dirty work. They do the work that no one else wants to do because it's too difficult, or to hazardous. And an army of Datas, all disposable, you don't have to think about their welfare, you don't think about how they feel. Whole generations of disposable people.
PICARD: You're talking about slavery.
GUINAN: I think that's a little harsh.
PICARD: I don't think that's a little harsh. I think that's the truth. But that's a truth we have obscured behind a comfortable, easy euphemism. Property. But that's not the issue at all, is it?
Here, Guinan employs the trick of Star Trek that it employs on it's audience. Piccard is unable or unwilling to address the reason why the argument that data is a machine is convincing, but still wrong... he has an emotional respect for Data as part of his crew and cannot see him as anything other than a person, but cannot help but realize that Data will be seen by strangers as a machine first. Guinan chooses her words to carry that line of thinking to the conclusion that Data would be quite important feature of society if this was to be his legal status, and while she demurs from the conclusion Piccard comes to, it only further settles Picards problem with the argument. As he argues in the trial, Data is a machine, but the question of his rights doesn't hinge on what his physical form is, but on capabilities he hasn't demonstrated in the court, such as keeping mementos of relationships he had with his crew, his decorations for service, his very quest for his rights, as evidence that data is aware of what his individual value to the crew is and is capable of signifying that relationship in these ways and more.
This is all because Guine provided a decouple of the emotional investment in Data for both Picard and the audience (who at this point, were seeing Data as one of the more popular show characters) and instead put the line of logic presented against Data to something that both the audience of Americans in the mid 1980s and Picard both view as a historical black mark on human history, slavery... the forced work of people society deemed lesser then they. It's timing is placed well towards the conclusion that the audience is already emotionally invested in Data is a person, but may not know why, other then they love him too much to let him leave. But as Picard comes to realize, that the emotion about Data's respected character masques the logical moral wrong that makes the stakes so much higher and the audience worried for their robot buddy: We know Data is more than a mere tool or computer or beast of burden but the we don't know that not everybody sees him as we do... and by relying on the emotional attachment to what we feel is a person, by declaring him property, we make him a slave and then the people who don't see him as a person will never come to respect him. It's hitting the audience over the head, but in story, Guinen's pushing the idea, is removing Picard from what he knows about Data the person, to see Data as a mass produced comodity and when this happens, the last time humanity did that to something like Data came straight to mind. It also helps the audience, who may know that Guinen was created after Whoopi Goldberg begged for a part, any part, on the revival of Star Trek (at the time, a valuable actress who Trek's budget couldn't afford). Goldberg's request was motivated because she owed her success to the orginal series. As a young girl growing up in the civil rights era, seeing the character of Uhuru, a black woman, on the bridge of star ship and a respected member of the crew, Goldberg realized that she could be an actress on TV and not have to play a maid. Uhuru was very important to many African Americans in the 1960s and not only inspired Star Trek fans into acting, but it was not uncommon for African American Astronauts to cite her role in their lives for why they decided to become Astronauts in the first place. Audiences' awareness of this fact in the episode brings them to see just how important what Goldberg was saying was. Her character wasn't playing Data's part... but she had been in that same role only two decades prior to the first airing of the episode.
If there is an emotional appeal, it needs to be the end of the letter, not the begining. Begin with why you wrote the letter, and present the facts of the case. You want to set it up so that by the time you make your emotional appeal, you've primed them to know the reason behind the emotion.