I don't think we disagree regarding OT treatment of false prophets. Where we may disagree is how that applies to common believer witnessing after the closing of the cannon.Well the 1st thing I think of is that we are not under Old Test. Law.
The 2nd thing that comes to my mind is the "reason" behind the death penalty for false prophets. Specifically when reading the text from Deuteronomy 13 we need to understand the context of the closing verses of the previous chapter (12) in which Moses reminds the people that God was driving out the current occupants of the land not just for worshipping false gods, but for all the detestable things done in service to these gods.
In particular, he mentions that the Canaanites would burn their children alive in the fire of Molech as a sacrifice to these gods (Dt 12:29-31).
In Leviticus 18, Moses relays God’s warning to the Israelites against imitating the practices of the Canaanites. He then lists several forbidden activities such as incest, adultery, homosexual acts, temple prostitution, child sacrifice, and bestiality. Archeologists such as William Albright have uncovered writings that detail the adventures and pastimes of the imaginary deities. So, when the Canaanites were participating in the above acts, they were merely imitating the perceived exploits of their FALSE PROPHETS for their false gods.
Then there is also another reason and it is related to Israel’s type of government. The nation was created as a Theocracy under the Mosaic covenant at Sinai with the true God as their Sovereign Ruler. Over the next four hundred years, Moses, Joshua, and a series of judges guided Israel as God’s chosen servant leader. Even under the monarchy, the various kings were themselves subject to God’s sovereign rule in what became a modified theocracy. As a result, the false prophet’s actions essentially amounted to the highest form of treason, which was a capital offence even in most modern human-government countries. How much more serious is this crime when committed against the true infinitely Holy God.
Then there is the one that I personally find the most acceptable. The teachings of those False Prophets was an act of MURDER and God said thou shalt not Murder.
You see, by leading people astray, the false prophets condemned their followers to an eternal death and separation from God in hell. Thus their actions were the same as committing spiritual murder. We could note here that Satan himself was the original false prophet. By giving a false message to Adam and Eve in Genesis 3 (“Did God really say…”), he condemned all mankind to physical death, and all unbelievers to spiritual and eternal death.
You can find much to disagree in society and you can spend much effort and heart-ache going to great lengths to confront evil, even though we know that society will continue to get worse and worse until He comes.
Or, you could adopt the approach oh Paul at Mars Hill (Acts 17). He could have used TRUTH to confront and condemn them, but he approached them in love and talked to them as at least potential friends.
Many did reject his teaching, but some accepted Paul's message.
How many do you think he would have convinced through condemnation?
The Truth is narrow when observed from the world, but His kingdom is truly wide. It has many aspects and some are more likely to open the ears of individuals.
You can choose to batter your head against on a stuborn unbeliever, who refuses to heed your understanding or you can look for a way to help them listen to the leading of the Spirit.
Few may accept the Truth.
Fewer still will respond to a frontal assault.