how is iago responsible for each death?

If you remember, it was Iago who framed Desdemona to make
it look as though she was having an affair with Cassius.  Othello murdered his wife
Desdemona because he didn't believe her innocence.  As it was Iago who made up the
affair, he could be blamed for Desdemona's death for this
reason.

Othello was overcome with guilt after realizing
that he had murdered his innocent wife.  Again, Iago was responsible for developing the
story that led to this death.

Emilia stole the handkerchief
from Desdemona as Iago requested, which was the "evidence" that Desdemona was having an
affair.  When she confesses this at the end of the play, it is Iago who murders her, and
he is obviously responsible for this death.

Hope
that helps!