Matt5:17 “Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. Gal3:19 What purpose then does the law serve? It was added because of transgressions, till the Seed should come to whom the promise was made; and it was appointed through angels by the hand of a mediator.
You are conflating two different things that are not related. The law is a general term and we have to read context to what it is referring to.
In Matthew 5:17 Fulfill does not mean the opposite of what Jesus just said, I did not come to destroy (end) and that is not in context with the passage, or the prediction of what Jesus would do with God's law.
Fulfill means to fill-full.
For example, when one fulfills their wedding covenant does that mean they are free to commit adultery or does that mean they are fulfilling their wedding covenant by being faithful. This is what Jesus is talking about- we are to fulfill the law by keeping it- If you love Me, keep My commandments- John 14:15
Which is why the rest of the passage in Matthew 5 does not fit your context that we are free to commit sin now.
Matthew 5:19 Whoever therefore breaks one of the least of these commandments, and teaches men so, shall be called least in the kingdom of heaven; but whoever does and teaches
them, he shall be called great in the kingdom of heaven. 20 For I say to you, that unless your righteousness exceeds
the righteousness of the scribes and Pharisees, you will by no means enter the kingdom of heaven.
21 “You have heard that it was said to those of old, ‘You shall not murder, and whoever murders will be in danger of the judgment.’ 22 But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment. And whoever says to his brother, ‘Raca!’ shall be in danger of the council. But whoever says, ‘You fool!’ shall be in danger of hell fire. 23 Therefore if you bring your gift to the altar, and there remember that your brother has something against you, 24 leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift. 25 Agree with your adversary quickly, while you are on the way with him, lest your adversary deliver you to the judge, the judge hand you over to the officer, and you be thrown into prison. 26 Assuredly, I say to you, you will by no means get out of there till you have paid the last penny.
27 “You have heard that it was said to those of old, ‘You shall not commit adultery.’ 28 But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart. 29 If your right eye causes you to sin, pluck it out and cast
it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell. 30 And if your right hand causes you to sin, cut it off and cast
it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.
Does Jesus say He has fulfilled the law that we can commit murder or adultery now? Of course not, He is magnifying God's law as predicted Isaiah 42:21 which means to make greater, not smaller. And notice this verse:
Matthew 5:19 Whoever therefore breaks one of the least of these commandments, and teaches men so, shall be called least in the kingdom of heaven; but whoever does and teaches
them, he shall be called great in the kingdom of heaven.
What commandments is Jesus talking about? The Ten, so I would consider these Words from our precious Savior.