Thou Fool

Matthew 5:21
You have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgement
v2 22
But I say unto you, that whosoever is angry with his brother without a cause shall be in danger of the judgement: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou Fool, shall be in danger of hell fire.

"Raca" is Hebrew and an expression of murderous anger.
Calling a brother in Christ stubborn,bull headed, without sense, having judgment that is always wrong, degrading comments without regarding another's thought patterns are all examples of "Raca".

Why is God so hard on this?

Because a saved person is a child of God, the image of Christ in the Father's eyes. When you come against a person with Christ living in them, you are taking on Jesus himself.

Criticizing comes from an evil heart, one that is in rebellion against God. The spirit of the anti-christ lives in such a person. Unjust anger is the same as murder, because the attacking person is killing Christ again.

Anyone rebelling against God has an evil heart. They may get away with this crime here, in modern day Babylon, but will never beat the wrath of God in His court.

This is clear because Jesus brings up the conversation about this issue and calls it murder. Love one another, don't criticize a brother in Christ. Be reconciled and use the word of God as your filter in seeing others.