“Either make the tree good, and its fruit good; or make the tree bad, and its fruit bad; for the tree is known by its fruit. You brood of vipers! How can you speak good things, when you are evil? For out of the abundance of the heart the mouth speaks.
If someone were to open their mouth and immediately speak problems they do so because it is in their heart. According to Proverbs there will be only death, destruction and sickness; nothing good will come out of it. Matthew 12:34 is aimed at the Pharisees and why did Jesus say that? The Pharisees had the scriptures which they taught to others but Jesus called them evil. They used scriptures to condemn and punish, they used the scriptures selfishly.
The Pharisees brought a woman caught in adultery to Jesus, according to the scripture she must be stoned to death. If Jesus said yes to stone her, these very Pharisees would report to the Romans that Jesus was interpreting the law, if Jesus says no then he would not follow the law of the scripture. Jesus used scriptures not to kill but to forgive her and change her life. Jesus used scriptures for life not death. The Bible says the elders left first, Jesus had the authority to condemn her because He did not commit sin but He forgave.
The good person brings good things out of a good treasure, and the evil person brings evil things out of an evil treasure.
If we speak only bad we cannot expect the good to be manifested in our life. We are supposed to bring good in our life by filling our heart with the Word of God.