In Act Four of "The Crucible" why does Hale say, "There is blood on my hands!"?

In act four, we see that Hale has experienced a change of heart.  In the first three acts, he was a part of the sentencing of many women, and a part of a lot of the questioning, condemning, and accusations that were a part of the witch trials.  But in act three, as John, Giles and Franics attempt over and over to prove the accusations false, and to valiantly save their wives and friends, Hale is touched.  He is especially moved by John's willingness to besmirch his reputation (regarding adultery) in order to prove Abigail Williams a cold-hearted girl seeking revenge.  After John confesses, and Elizabeth lies about the affair, Hale finally jumps in and states,

"I may shut my conscience to it no more-private vengeance is working through this testimony!  From the beginning this man has struck me true...I believe him!"

Later, he even goes as far as to declare, leaving, "I quit this court!"  He is so disgusted with the fact that the judges believe Abby's antics that he leaves, frustrated.

In the interim between acts three and four, we are led to believe that Hale has been approaching each of the condemned prisoners and begging them to confess, so that they won't be hanged.  He doesn't like this task; in fact, he calls it "the Devils work" because he "comes to counsel Christians they should belie themselves", meaning, lie about being a witch, in order to save their life.  But, he feels guilty that he condemned so many people to hang, so he is trying to save as many lives as he possibly can.  He feels personally responsible for their deaths.  So, when he says, "There is blood on my head," he is referring to the fact that it was he himself who signed their death warrants, and so their blood, or lives, is his personal responsibility.  He bears the blame, weight, or fault, of these people dying innocently, on his shoulders.  Saying that there is blood on his head is just another way of him saying that he feels guilty for the death of these people.  I hope that explanation helps a bit; good luck!