Othello says this speech to Gratiano--at least initially--after Emilia has told him the truth about the handkerchief and Iago has killed her in retaliation.  Othello is being held prisoner in the bedroom while the others are hunting for Iago.  Gratiano is standing guard outside the door.

Just before this speech, Othello has asked Gratiano to step inside the room.  He confesses to Gratiano, who thought Othello was disarmed, that he has a weapon that any soldier might use to commit suicide.  And with this sword he has fought many men and could easily take Gratiano.  But he disdains boasting about his prowess and feels helpless in controlling his fate.  He declares that he is at the end of his journey and will do no harm to anyone else.  There is no need for Gratiano to be afraid; Othello claims he could be knocked over by something as flimsy as a reed.  If he should die, he knows he will go to hell.  If he should to heaven, one look from Desdemona would hurl his soul out of heaven into hell.

He turns to Desdemona and touches  her dead body and admits that she is cold and pure.  For his crime of murdering such an innocent, he feels that he cannot suffer too much.  He wants o be roasted, burned, or blown about.  No punishment is too great.