Othello's last speech is a culmination of his other words in that act-- it is obvious he feels extreme remorse and has realized the error of his ways. He knows his own inglorious death is near and that he has made a monumental mistake. He says:

Of one not easily jealous, but, being wrought,
Perplex'd in the extreme; of one whose hand,
Like the base Judean, threw a pearl away
Richer than all his tribe; of one whose subdu'd eyes,
Albeit unused to the melting mood,

Making the audience understand that he realizes that he has killed something beautiful, and hates himself for it. It's hard not to feel at least a little sympathy for someone who has been manipulated and recognizes the error of his ways. This is a very neat trick, as we've just seen Othello brutally murder Desdemona. The fact that we can now feel sympathy for him is a testament to the power of his speech that emphasizes his own self-hatred.