This question is referring to the crucible What “law” is Danforth referring to in his assertion: “I should hang ten thousand...

The "law" Danforth refers to is Salem's Religious law. 
Remember, during this time period there was no separation of church and state.  The
rules of the state were the same as the rules of the church.  Think back to the
beginning of the play in which Miller's exposition tells us that people were charged
with walking around the town on Sundays to make sure people were in
church.

In the instance of this quote, Danforth believes he
is doing God's work by eradicating Salem of the witchcraft threat.  This includes
fighting those who are against him, and therefore also against the work of the church. 
It also shows that he and the other judges are cruel and refuse to listen to any
evidence that goes against the girls’ testimony or the court’s
case.