"The play 'Amadeus' is mainly concerned
with the destructive nature of jealousy"
This passage is all too true, both in Peter Shaffer's 'Amadeus' and in life in general. However the play is also concerned with the destructive nature of ignorance and naivety. Salieri is jealous not just of Mozart's talent, but of the fact that God gave the talent to "Mozart ... spiteful, sniggering, conceited, infantine Mozart". He is envious of the vessel of God's laughter at the 'patron saint of mediocrity' as he had dubbed himself. Not only did God double-cross Salieri, but he did it using this "obscene child". It was this jealousy and the rage it inspired that caused Salieri to attempt to kill Mozart by starving him of work and students, and thus, money and food. If Salieri had not restricted the amount of work actually shown to the general public, then Mozart could have been wealthy, and quite possibly selected as the new Kapellmeister.
Mozart doesn't understand the importance of pleasing members of the Viennese court. He has no comprehension of the value of money, for when he successfully earns any, he spends it on lavish food and clothes immediately, instead of saving it. He spends all his time churning out music in final copy, which, although beautiful, doesn't earn money as would teaching music.
Mozart is really the one who should be jealous, as he has little in the way of money or assets, or even respect. All he has is his talent and his priceless music, but not the sense he needs to capitalise on it.
It isn't the fact that someone has more talent than him, the problem is, as he sees it, that he isn't as good as someone else after making a bargain with God, which should guarantee that he is the best in the world at least until he dies. But no. "I know myself forever mediocre" he cries as he realises that the gift given to him by God only exists to allow him to recognise the greatness and incomparable beauty of the music of God. It is at this moment he resolves to condemn God in everything he does. He cancels all his appointments to help poor musicians, to break his vow of social virtue. He then seduces Katherina Cavalieri to break his vow of sexual virtue. He makes the conscious decision to destroy God through Mozart in everything he does. He declares God, through Mozart, to be "Nemico Eterno", the eternal enemy. His life is now devoted to the destruction of God.
I believe that Antonio Salieri is also jealous of God's ability to dole out talent as he sees fit, without Salieri being able to stand in his way. He has had enough! So he snaps, and attempts to destroy Mozart's name and reputation, which wasn't that high to start with. His name wasn't incredibly well known because only Salieri had the ability to recognise his greatness. Because the rest of the Court, and indeed, Vienna, relied almost solely on the opinions of Herr Salieri, he was able to prevent Mozart's music from becoming popular to a degree. He won his fight with God, but the knowledge lived in him still, he knew he would forever be, the patron saint, of mediocrity.