We generally read Scripture with the eyes of the year we live in. It's in forcing ourselves to read with Hebrew eyes that we find what are gold nuggets to us, but easy to understand statements to the contemporaries of the Messiah.
When it comes to Matthew 5:17 I'm a Bahnsenite. For those of you who don't know what that is, you probably should skip this blog post and go read Bahnsen on law. You've got Google so that's all I'm saying.
I've read, and re-read the above Biblical text too many times to count. I've also read Bahsen exegete the text in his works. I've also read Bahnsen respond to his critics on the text. For the lover of the law (Psalm 119:97), it's fine wine.
But... I think the best way to understand the text is to realize its first century context.
We read Yeshua here as saying, "Don't think I've come to destroy the law," and we immediately think he's saying something like, "I've not come to throw the law in the garbage." That's good. I think that is a true thought, but I've come to believe that the Lord had something else in mind when he made the statement. Something his immediate listeners would have caught right away.
The various rabbi's (teachers) in Israel in the first century often discussed how the Torah (Biblical law) was to be carried out. In those discussions, ideas were bounced back and forth. How do we properly keep this commandment? What is the meaning of that ordinance? What do we do or not do on the Sabbath? These are the matters that were discussed on a daily basis.
When one rabbi thought another rabbi had misinterpreted the Torah, the common way to speak of this was "the Torah has been destroyed." If it was believed the rabbi had properly interpreted the Torah, the Torah was said to "have been fulfilled."
Yeshua was explaining to the crowd (Matthew 5:17), "Don't think that I've come to misinterpret the Torah. I've come to explain it accurately."
This helps tremendously in the remaining verses of Matthew 5 where the Messiah contrasts the teachings of the scribes and Pharisees (vs. 20) with his own. Each of his statements begin with, "You have heard that it was said to those of old" (vs. 21) followed by a "But I say unto you" (vs. 22). What Yeshua is doing is giving the predominant, Pharisaical way of interpreting a commandment in Torah and then contrasting it with his inspired interpretation.
In doing so he wasn't destroying the law, he was fulfilling it. He was in essence showing that in many areas it had been destroyed by the scribes and Pharisees. He was coming to "prop it back up" or explain to the masses its true intent.
One example of this will be sufficient.
The scribes and Pharisees were apparently quoting "Do not murder" (Matthew5:21; Exodus 20:13) but only applying it to the act of physically taking someone else's life, intentional manslaughter. Yeshua says that murder is more than that. He explains that insulting outbursts, or hatred in one's heart towards his brother was murder (Matthew 5:22). I think it's safe to say that the Lord brings this up because the Pharisees had neglected to apply the commandment to the sins of the heart. They were guilty of hatred in their heart, yet justified their actions by interpreting the law in a way that only found one guilty if the physical act of murder was carried out. Many people have refrained from committing physical murder. We're all guilty of the other.
If we seek to understand the true intent of the law, and have it do to us what it is primarily supposed to do (show us our sin), we must sit at the feet of Rabbi Yeshua who came, not to destroy (misinterpret) the Torah, but to fulfill (correctly interpret) it.
 Evidence of this is found in David Bivin's work, "New Light on the Difficult Words of Jesus" pp. 93-102.