He eventually tried to ruin Mozart's life to benefit himself in becoming the best composer, but, to his horror, was replaced by Ludwig van Beethoven, played by Nelson. This final setback caused Salieri to go insane and willingly institutionalize himself.
Behind the Laughter
As pointed out by Lisa, Marge's story isn't correct. In real life, Salieri and Mozart were not from the same family. They had an intense rivalry, but sometimes they were seen as friends and colleagues and even supported each other's work. She also points out that Mozart was a hard worker and Salieri was celebrated in his time. The real Salieri was also Italian.