And He's addressing the Jews, not the body of Christ. This is not Church doctrine. Or do you believe you can lose your salvation by calling someone a fool?
I don't believe that Jesus is talking about believers losing their salvation over a "heat of the moment" situation of calling someone a fool. Murder and hatred here stems from ongoing sinful motives from an unregenerate heart. In context, Jesus mentions the law and in verse 20, Jesus said - "For I tell you that unless your righteousness surpasses that of the scribes and Pharisees, you will not enter the kingdom of heaven." That statement from Jesus probably came as a shock to the multitude of people who had great admiration of the scribes and Pharisees for their knowledge of the law and seeming righteousness and holiness in external observance of the law.
The righteousness of the scribes and Pharisees was mere "outward conformity" to the law (coupled with adding their own traditions to the law) yet Jesus points to inward conformity. He says, “You have heard it said, but I say unto you” to differentiate between the way people heard the law taught from how Jesus is reinterpreting it. Obeying the law is more than simply abstaining from physically killing someone, physically committing adultery, breaking oaths etc.. It’s also not getting angry with your brother from a hateful heart, not lusting in your heart, not making insincere oaths etc..
The righteousness that exceeds the "righteousness of the scribes and the Pharisees" (self righteousness) is
the righteousness of God which is from God by faith. Philippians 3:9 - and be found in Him,
not having my own righteousness, which is from the law, but that which is through faith in Christ, therighteousness which is from God by faith.