As act 5, scene 2 opens, Iago has convinced Othello that Desdemona is having an affair with Cassio. This is a lie: Iago has plotted and planned and manipulated Othello into believing in an affair that is not true. Othello gets so frenzied that he kneels before Iago and swears he will have revenge on Desdemona for her betrayal.
At the beginning of act 5, scene 2, Othello enters the bedchamber where Desdemona is peacefully asleep. When he says "it is the cause, it is the cause," he means that killing her is the reason he is there: it is what he has to do.
However, as he leans over her sleeping body, he realizes he still loves her. She seems as pure as alabaster. He cannot strike right away. He steels himself to the task, saying:
Yet she must die, else she’ll betray more men.
He says he will put out the light—the candle by the bed—and then put out the light of her life. He understands, however, that once she is dead, he can't relight her as he could a candle. This is the end. She is a rose that will wither. There will be no going back.
As he feels her breath, he almost changes his mind, but he feels he must not weaken. He must take his justice by killing her. His sorrow being heavenly means he feels he is killing her because he loves her, just as God punishes people out of love.
She looks so beautiful to him sleeping that he kisses her, and this wakes her up. They will go on to have a conversation in which he makes sure her soul is right with God, as he wants her to go to heaven. He does not believe her assertion that she is innocent.
The great irony of the scene is that while Othello believes he is doing the right thing, he is acting based on a lie—Desdemona is innocent of any wrongdoing. Whether adultery deserves death is another question, but Desdemona is not adulterer. Othello will find out too late that Iago betrayed him.