****Spoilers*********
This movie has many historical inaccuracies. There is not a strong evidence that Salieri had even plot to kill Mozart, though he confessed he had killed him when he went insane.

The real Mozart had even been nuttier than what's been portrayed. He is sacred and profane at the same time, that's actually why I love him so much.

Also The Requiem Mass was not commissioned by Salieri, if you follow the second link in my signature, you'd find great information on this fine piece.

I suggest you do more research on this matter, because this movie is only made to get you to know Mozart's music.