1)
Why did they say that he is Chayav Misah?
1.
Malbim: The Kohanim and [false] Nevi'im did not believe his Nevu'ah, and wanted to kill him according to law, for being a Navi Sheker. He prophesized the opposite of their prophecy! The Am did not believe this, but still wanted to kill him to remove the harm 1 to them from his prophecy of punishment.
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Did they think that this will evade the punishment?! Perhaps the 'harm' was making them more Mezid. Or, his prophesy was unpleasant to their ears. (PF)