"He that despised Moses' law died without mercy under two or three witnesses:" (Hebrews 10: 28)
Moses' law was a serious matter.
It was no light matter to belittle the law, since it was the ministry of condemnation and death, God's inexorable standard which no one can keep.
The Good News for those who believe on Jesus, however, is that Jesus not only respected the law, since He is the law-giver, but He fulfilled it for us, since no one can be justified by the law:
"Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfill." (Matthew 5: 17)
"For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. (Matthew 5: 18)
He fulfilled every demand.
Let us also be clear that no one can be justified under the law:
"Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin." (Romans 3: 20)
"Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified." (Galatians 2: 16)
When Jesus died on the Cross, He not only paid for all our sins, but put an end to the Old Covenant:
"13And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses; 14Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross; 15And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it." (Colossians 2: 13-15)
While no man received mercy for despising Moses' law, we can rest at ease knowing that Jesus was despised and rejected on our behalf, and He fulfilled God's law for us.