What does Rev. Hale mean when he says there is "blood on his head"?
This is actually originally a quote from the Bible, Kings 2:32,
The LORD will return his blood on his own head, because he fell upon two men more righteous and better than he and killed them with the sword
This quote is appropriate for Hale, who is a weak man and knows that the only crime Proctor is guilty of is adultery, yet he lacks the courage to question the decisions of the court.
He is acknowledging his responsibility in the deaths of those accused of witchcraft.