This last speech of Othello is his way of expressing to viewers how he would have liked them to see the events of the play. However, his speech, albeit elegant and characteristic of Othello's extravagant and Romantic use of language, is flawed, ironic and thus it is impossible to see the events of the play in the light that Othello would like us to. His account is merely how he would like the audience to view the play and not necessarily how the events of the play have indeed unfolded.
Othello is a man very conscious and in some ways obsessed with reputation. He himself is aware of this, as shown in the preceding lines: "I pray you, in your letters, / When you shall these unlucky deeds relate, / Speak of me as I am". Aware of the importance of opinion, which is inevitably affected by one's reputation, he quickly jumps to tell people of how he would like to be remembered.
With this obsession with reputation, it is already clear that Othello has a strong motive to put a positive spin on his actions. Indeed, his preoccupation with his reputation and public image was one of the reasons that he killed Desdemona. Also, it is Iago's reputation as an honest man which allowed him to deceive Othello without Othello once questioning his words. Thus, Othello has a strong motive not to portray himself as he really is, but how he would want us to see him and so it is already possible to see that his account of his behaviour may not necessarily be an accurate one.
The preceding lines state that he tells the others to "speak of me as I am". However, there is great irony in this statement as he immediately goes on...