Verrrry interesting that you mention John Piper. Here some articles where John Piper said Paul was a murderer:
https://www.desiringgod.org/articles...ome-a-murderer
https://www.christianpost.com/news/j...-christ-69994/
It's very simple. If Paul was a murderer then He broke God's laws. That's why he needed Christ. Paul was never ever sinless in the eyes of the law, neither old testament or new testament law.
By the way, another example of a murderer is David. David conspired to kill Uriah, yet Nathan said David himself struck him down:
2 Samuel 12:9 ".....You struck down Uriah the Hittite with the sword and took his wife to be your own."
You don't seem to understand that conspiracy to murder is the same as murder in God's eyes. It doesn't matter whether Paul or David actually held the knife or not, it's still murder.
Also, the "higher standard" of the New Testament was actually Jesus explaining the full, spiritual meaning of the Old Testament Law.
Matthew Henry commentary:
5:21-26 The Jewish teachers had taught, that nothing except actual murder was forbidden by the sixth commandment. Thus they explained away its spiritual meaning. Christ showed the full meaning of this commandment; according to which we must be judged hereafter, and therefore ought to be ruled now.
You are talking like one of those Pharisees when you disagree that Paul was a murderer:
What does it prove? Threatening murder means murder?
You know yourself there is no reference of Paul committing direct murder in the scripture. That's why you have to refer to what makes him "a murderer in God's eyes (even hatred is murder according to Jesus)."
The concept of "direct murder" versus "indirect murder" is a Pharisee concept. Jesus made clear that all forms of murder, even hatred, is murder in God's eyes.
This is why I said that Paul was a murderer. John Piper agrees with me.
You are clearly wrong when you said
"He still claimed he was sinless according to the law at that time. Unless he was lying, he did keep the law."
Paul was never sinless according to the law in God's eyes. Whenever his heart started to hate a Christ-follower he became a law breaker as a murderer.