But it doesn't. Abraham was sent to kill Isaac. Many times God commanded Moses and other prophets to kill people.
When I used to go to church people would always talk about wanting to be prophets like Elijah. Elijah killed 500 (I think) prophets of Baal. Elisha, his apprentice once cursed some children who were then slaughtered by bears;
So my question still stands. And I think it's a valid one.
Abraham did not kill his son. A curse does not kill either, but God causes it to happen. Moses and the prophets who ordered the deaths of others did so because of something those people did wrong, and because they were harmful for their society. It would be different today, when we have prisons and law enforcement to contain and handle criminals. If God did order someone to be killed, it would have to be for a very good reason.
Given this (and I suspect most Christians would agree with what you say here), do you think that there's reason to believe Abraham may have been deceived by a demon, or that he was possibly mentally ill? What about other people in the Bible; because they often heard God's voice too? How do we determine whether someone is mentally ill or hearing from God?
No. Abraham knew God's voice. He had heard it before when God promised the birth of Isaac. And before that, he heard it when God announced the destruction of Sodom and Gomorrah, which allowed him to go and save Lot.
The Bible also tells us that Abraham expected that God would raise his son from the dead, since Isaac was the son God had already promised He would make a great nation out of. He didn't think that God would have him slay his son, and that that would be the end of it.
A person who is mentally ill will have symptoms of their illness. Plus, we now have the technology where we can look at the brain and see if its physical structure resembles, say, schizophrenia.
If someone is hearing from God, then what they say will:
1. not contradict God's word, as it has already been given to us.
2. bring predictions that come true.
3. be consistent with the personality of God, as one who is loving of His people but hateful of sin.