Malbim: Initially, the nation joined with the Kohanim and wanted to kill him. This is not because they thought that he is a false Navi. They believed that Hashem sent him! According to letter of the law, he is not Chayav Misah. They wanted to kill him not according to Torah law, for they did not want someone rebuking them and telling them Devar Hashem, since they did not want to hear Devar Hashem.