What does Hale mean when he says, "Why, it is all simple. I come to do the Devil's work...Can you not see the blood on my head!"?
When Reverend Hale returns to Salem, he does so with a great deal less confidence and authority than when he came for the first time. In Act Four, he tells Elizabeth Proctor,
I came into this village like a bridegroom to his beloved, bearing gifts of high religion; the very crowns of holy law I brought, and what I touched with my bright confidence, it died [...].
He understands, now, how he contributed to the hysteria, then. He knows that he has played a role that contributed to the deaths of innocent people and which will go on to contribute to the deaths of several more...unless they confess a lie and save their lives. To this end, he tells Elizabeth,
Life, woman, life is God's precious gift; no principle, however glorious, may justify the taking of it. I beg you, woman, prevail upon your husband to confess.
John Proctor, we later learn, is torn between maintaining his integrity and being hanged as a result, and relinquishing his integrity and keeping his life; he eventually chooses to die and keep his integrity. Hale, however, believes that it would be better to tell the lie and keep one's life than to proudly hold on to sinlessness in the face of such injustice. He feels the weight of his own responsibility and guilt in the deaths of those innocents who have died before, and he feels that he is doing the "Devil's work" by advocating the telling of lies, but he is, in essence, choosing the lesser of two evils.
When Hale delivers the lines you've cited, he seems to feel a mixture of guilt and responsibility, judgment of the magistrates (to whom he directs his "sarcasm"), and desperation to save the lives of the innocent people condemned to die.
Here is the full quote for reference:
"Why, it is all simple. I come to do the Devil's work. I come to counsel Christians they should belie themselves (his sarcasm collapses)There is blood on my head. Can you not see the blood on my head!"
Reverend Hale has returned to Salem at this point in the play, Act IV. He feels a great deal of guilt about having contributed to the hysteria in Salem regarding the accusations of witchcraft.
He specifically returns to Salem to convince the remaining accused to confess, and thus save their lives. He feels the weight of those who have already been executed on his own head. He feels a deep responsibility for his contribution to the deaths of others.
The blood he refers to is the blood of the innocent who have been put to death.
Reverend Hale goes through a transformation in the play, at first enjoying his role as authority on witches, but then he feels great remorse and guilt due to the lengths the people in Salem go to rid their town of "witches."
Hale has come to believe that the accused are innocent. Lying, or "bearing false witness" was a sin that condemned to hell. He is, however, telling the accused to lie and say that they are witches to save their souls. By doing so, he is giving his soul, and theirs, to the devil.