Othello was certainly responsible for Desdemona's death. Although we can see that he was cruelly manipulated by Iago, he still has a choice as to his actions towards Desdemona.
Also, her killing is premeditated: he plans the means by which she will die on Iago's suggestion, and tells Desdemona to wait for him, alone, in her chamber.
Thirdly, Desdemona pleads for her life, protesting her innocence to the death. Othello has formerly seen her testimony as truthful - he lays his life on her testimony regarding the circumstances of their courtship and marriage as explained to the Duke. When the situation is reversed he refuses to listen to her.
1. Not only does Othello physically smother Desdemona; he also does nothing to help her when she briefly becomes conscious again.
2. Othello allows Desdemona to accompany him to a rough military outpost (Cyprus) which isolates her and provides her with no family support (to refute the allegations against her).
3. Othello completely trusts Iago's word over his wife's; yet Iago was originally someone whom Othello did not think highly enough of to appoint as his lieutenant. This poor judgment on Othello's part ultimately leads to the general's choice to kill his wife.
4. One final point, when Desdemona drops her handkerchief, Othello tells her to leave it behind. Why could neither he nor Desdemona remember that that is when the handkerchief goes missing--this could have prevented almost all of the deaths.
The obvious point to make is that he is the one who actually suffocates her. The other major point that I can think of is that Othello is the one who choses to believe the lies that are spread about Desdemona. If he had simply trusted her, he would not have been inclined to kill her. Therefore, it's his fault.
I think that Iago should be the accused for her death because he was the one to incite Othello to kill her. The quote should be in Act 5 scene 1