There is more mercy in Christ than sin in us

"Or have you not read in the Law how on the Sabbath the priests in the temple profane the Sabbath and are guiltless?  I tell you, something greater than the temple is here.  And if you had known what this means, I desire mercy, and not sacrifice,’ you would not have condemned the guiltless.  For the Son of Man is lord of the Sabbath.”

Why would Jesus have confronted the keepers of the Law and the Temple with these words? Did God not command the sacrificial system? 

It's rather simple: 
We can offer our sacrifices without fully understanding the mercy that was given to us. Jesus was clear on this when he spoke of the debtor who's debt had been cancelled and did not show the same mercy (Matthew 18:28ff).

If we know that we have been spared from the wages of sin, which is death, then we will live lives of mercy. 
We will offer forgiveness to those who kill us. 
We will not slander those who slander us. 
We will give to those who cannot pay us back. 

Simply because that is the way of God.  

"There is more mercy in Christ, that sin in us"