Michael Cassio was unfairly treated as a result of Iago's manipulation. Iago hated him because he had been appointed as Othello's lieutenant, a post he desired and felt he was entitled to since he had been so loyal to Othello. Furthermore, to add insult to injury, Michael was an outsider, a Florentine, and according to Iago, he had no military knowledge or experience and had only book knowledge. Iago felt insulted and humiliated that Othello dared to appoint him and swore revenge on both.
Iago also resented Michael's good looks and charm. He was young and had a way with women, whilst he had none of those qualities. Iago was, in contrast, manipulative and scheming. He, therefore, felt it his duty to do whatever he could do hurt the unsuspecting lieutenant. It was for this reason that he plotted with Roderigo to have Cassio, firstly, dismissed, and secondly, killed.
What made Iago's resentment and treatment of Cassio unfair is that Cassio did not choose to be what he was. The traits he possessed were ones he was born with. Secondly, it was not his decision that Othello chose him as his lieutenant. He only applied for the position and was appointed. Whatever reason Othello had for commissioning him was the general's decision and Michael could not be held accountable for that.
Cassio's only mistake was the fact that he did not resist Iago's urge for him to take another drink. His resolve was not strong enough even though he knew he had already had one drink and did not have the constitution to resist the effects of the intoxicant. By the time he took the second drink, he was already a bit drunk. Iago wanted him to imbibe so that he would be easily affected by Roderigo's taunts and his challenge.
This was all part of Iago's scheme. With Cassio intoxicated, it would be easy for Roderigo to provoke him into doing something wrong and earn Othello's ire. Iago had already prepared the way forward by telling Montano that Cassio had a drinking habit. He drank every day before going to bed. Montano then said that he should inform Othello about Cassio's weakness, but Iago ironically asserted that he would never harm Cassio.
Iago's plan worked perfectly. Cassio went off to bed insisting that he was not drunk. On the way he was confronted by Roderigo who picked a fight with him. The obviously inebriated Cassio reacted and the two men started a brawl. They re-entered the scene and Cassio threatened Montano when he intervened. The two men fought and Montano was seriously wounded. Roderigo ran off at Iago's instruction and raised the alarm, waking Othello in the process. On arrival Othello asked Cassio:
How comes it, Michael, you are thus forgot?
Othello wished to know how Michael had lost his composure. Since he was Othello's lieutenant, he was supposed to keep the peace and be well-behaved, given the conditions in Cyprus with the threat of an attack from the Turks. Cassio was too overwhelmed to speak and said:
I pray you, pardon me; I cannot speak.
On his inquiry about how the brawl began and who had been responsible, Iago gave all the details about what he had witnessed. Othello, without giving Michael an opportunity to defend himself (probably because he had become mute from shock) summarily dismissed him saying:
I know, Iago,
Thy honesty and love doth mince this matter,
Making it light to Cassio. Cassio, I love thee
But never more be officer of mine.
And thus, the innocent Michael Cassio becomes a victim of Iago's evil deception. His attempts to counsel with Desdemona (on Iago's advice) in order to regain his position, were slyly used by the evil master manipulator to convince Othello that Michael and Desdemona were having an affair making Othello a cuckold. Othello and Iago later make an oath to kill the two supposed lovers.