Why did Ibn Taymiyyah issue a fatwa for killing whoever declares their intention (for prayer) aloud?

The words of Sheikh al-Islam are not about vocalizing the intention, but rather about the danger of saying, "Everyone does in his religion what he desires." He states that whoever insists on this phrase after proof has been established against him, or on any innovation, or attributing falsehoods to the Messenger, should be asked to repent, otherwise he will be punished. In fact, persistence in it may necessitate execution, because it is not permissible to act in religion except according to what Allah and His Messenger have legislated, and not according to what a person desires and follows of whims. Allah Almighty says: {And who is more astray than one who follows his own desire without guidance from Allah?} (Quran 28:50), and He says: {But no, by your Lord, they will not believe until they make you judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and submit in [full, willing] submission.} (Quran 4:65).