Below, J did a great job noting that the Beatitudes are focused on action not just rejection. This started life as a comment, but I decided to go ahead with a new post.
So often than not, we ascribe holiness to some sort of denial of action. Holiness is doing-not rejection. This was my favorite conversation last year with our two holiness guru's (leroy and summers).
If we look further in the S.O.M we see Jesus bringing up a few other issues that require simple denial under Jewish law (my bible files them under "concerning..." meaning 5:21-48), but He calls us to a further style of living. Each of these sections starts off with "You have heard it was said...". Last year I taught the S.O.M to a group of older folks in a Sunday School class. One of the issues that I wanted to handle with kid gloves was the section on divorce (5:31-32), because several of the people in the class had been divorced and remarried. All five of these sections note an element of Jewish law that was understood to be pretty important, almost basic for Judaism.
By Jesus starting off with "You have heard that it was said", he assumes that the crowd has a good working knowledge of these laws. So what he does is call them on doing things half-way!!! He takes these basic laws, which can be pretty hard to keep, and calls people to do even more. He puts these new actions in the plain of almost impossible, unnatural even.
This calls them (as us) to action. He calls them to holiness. I often think about the last verse of ch.5, and how it seems to be the perfect (yet utterly terrifying) end.
"Be perfect, therefore, as your heavenly Father is perfect" Matt 5:48
According to Lowe and Nida τέλός (perfect) is given the fuller definition of those who have been brought into a community of faith, and it pertains to completion have finished something. The perfect one has complete genuineness, and has no defect whatsoever.
What else is odd about this is how we see both the plural nominative τέλειοι (our perfection) and the singular nominative τέλειός (the Fathers character). Clearly this is how Jesus is ending this giant section on having an active righteousness that is the evidence of the kingdom being alive inside of the believer.