Murder could also be defined as an unjust killing of a human. Abraham questioned whether that would have been right for God to do.
@Right Divider said it was blasphemy to question whether God could murder, so it seems like Abraham is guilty of RD's blasphemy charge.
Again, that's not what Abraham was challenging God on.
And Abraham came near and said, “Would You also destroy the righteous with the wicked? Suppose there were fifty righteous within the city; would You also destroy the place and not spare it for the fifty righteous that were in it? Far be it from You to do such a thing as this, to slay the righteous with the wicked, so that the righteous should be as the wicked; far be it from You! Shall not the Judge of all the earth do right?”
https://www.biblegateway.com/passage/?search=Genesis 18:23-25&version=NKJV
Notice what Abraham actually said.
"Far be it from You!"
He wasn't accusing God of being capable of murder. He was appealing to God's character as the righteous Judge of all the earth.
I don't understand why your other questions, because they don't address the point where the conversation was.
Don't you think it might be important to know the context of a passage, including the way the ancients perceived things, when examining scripture?
Or do you think we should simply impose our modern assumptions onto an ancient text?
Ancient societies were largely organized around honor, shame, and reputation, rather than the modern tendency to frame every moral question in terms of abstract philosophical categories.
Abraham, as a follower of God, was concerned with what it would mean for God's name and reputation if He destroyed the righteous alongside the wicked, treating both alike.
Such an act would appear to contradict the justice of the very God Abraham worshiped.
That's why Abraham appeals to God's identity as "the Judge of all the earth."
He wasn't proposing that God might be a murderer. He was appealing to God's righteousness as the very reason God would not act unjustly.
And there's a rather significant difference between questioning whether God will act consistently with His righteous character and asserting that God is capable of acting contrary to it.
You're treating those as though they're the same proposition. They aren't.