Since Hill obviously knew that he would have to answer to God for his actions I don't think he was assuming God's place.
God instructs us that in earthly matters, we are our brothers keeper, and I think that Hill was assuming that role. Hill rightly thought that abortion was no less than unjustified murder, and since it is 'legal' he couldn't work within the system to stop it.
He did the only thing his beliefs left to him.
You besmirch all the Pro-Life people that obey the law. There is plenty you can do to fight abortion without becoming a murderer.