In Othello, why does Iago want to destroy Othello, Desdemona and Cassio, despite the fact that he already got Cassio fired?

Iago particularly wants to destroy Othello to seek revenge
for Othello's naming Cassio as lieutenant.  Iago felt certain that Othello would give
the title to him, so when Othello names Cassio instead, Iago is full of rage.  Further,
Iago thus becomes jealous of Cassio because he is now of a higher rank and is young and
handsome to boot.  Desdemona gets wrapped into Iago's evil plotting by mere
circumstance--Iago knows that Othello values Desdemona and their relationship above all
else, so Iago uses Desdemona as a tool in his plan. 

Iago
becomes overtaken by his own jealousy and rage, so he continues with his plans even
after Cassio is fired.  By this point in the play, Iago is no longer trying to get the
rank--he is almost obsessed with seeing Othello's fall.