Matthew 5:17-20 - Fulfilling the Law; Fulfilling the Prophets.
Yes there is a contrast between destroy(annul) and fulfill. And this can be likened to breaking and keeping a promise. However, both when you break and when you keep a promise, in both cases the contract ends. Only with the breaking, one's 'yes' becomes 'no,' and with the fulfilling one's 'yes' becomes 'yes.' After writing about how his intention to visit the Corinthians was thwarted even though he did not take his promise lightly, Paul assured the Corinthian congregation that "for no matter how many the promises of God are, they have become 'yes' by means of [Jesus]. Therefore, also through him is the 'Amen' said to God, which brings him glory through us." - 2 Corinthians 1:20
Jesus made it clear that now John the Baptizer finished preparing the way for Jesus that the time for fulfilling both the Law and the Prophets had arrived.
"The Law and the Prophets were until John. From then on, the Kingdom of God is being declared as good news, and every sort of person is pressing forward toward it. Indeed, it is easier for heaven and earth to pass away than for one stroke of a letter of the Law to go unfulfilled. - Luke 16:16,17
How was this true in the case of the Prophets? Here is one example:
"So the scroll of the prophet Isaiah was handed to him, and he opened the scroll and found that place where it was written: 'Jehovah's spirit is upon me, because he anointed me to declare good news to the poor. He sent me to proclaim liberty to the captives and a recovery of sight to the blind, to send the crushed ones away free, to preach Jehovah's acceptable year.' (Isa 61:1,2) With that he rolled up the scroll, handed it back to the attendant, and sat down; and the eyes of all the synagogue were intently fixed on him. Then he began to say to them: 'Today this scripture that you just heard is fulfilled." - Luke 4:17-21
Yes, now that the King designate was in their midst, and as he was bringing refreshment both physical and primarily spiritually he was fulfilling, or satisfying, the conditions of that promise.
Now that Matthew 5:17,18 has been covered. What of the rest of the passage? Here it is in full review:
"Do not think I came to destroy the Law or the Prophets, I came, not to destroy, but to fulfill. Truly I say to you that sooner would heaven and earth pass away than for one smallest letter of one stroke of a letter to pass away from the Law until all things take place. Whoever, therefore, breaks one of these least commandments and teaches others to do so will be called least in relation to the Kingdom of the heavens. But whoever does them and teaches them will be called great in relation to the Kingdom of the heavens. For I say to you that if your righteousness does not surpass that of the scribes and the Pharisees, you will by no means enter into the Kingdom of the heavens." - Matthew 5:17-20
Those that keep the Law and taught others to keep it as long as it was in force (not the hypocritically righteous), were the type of people that Jesus was willing to make a new covenant with at the end of his ministry, thus fulfilling the promise of Exodus 19:5,6. He did this on the night before his death. (Jer 31:31-34; Mt 26: 27,28; Mr 14:22-24; Lu 22:20; 1Co 11:25; Heb 8:8-12; Re 20:6)
"Now if you will strictly obey my voice and keep my covenant, you will certainly become my special property (or "treasured possession.") out of all peoples, for the whole earth belongs to me. You will become to me a kingdom of priests and a holy nation." - Exodus 19:5,6a
Yes, had enough of the Jews in Jesus' day been faithful in keeping the Law as it was intended - being led to recognize and honor the prophet of Deuteronomy 18:18 - all those that could have entered into that new covenent would have also been from those under Law. They would have completely filled the ranks of secondary kings and priests under Christ in God's Kingdom. No need would there have been to graft in those 'not my people,' to become 'my people' so as to fill the ranks. (Hosea 1:10)