Christians claim that Allah is Satan.
There are books out there about this:
Islam is of the Devil - Terry Jones
Satan's Biggest Lie: Islam - Michael J. Oldham
Islam and the Soldiers of Satan - Evang. Promise Baker
The reason they give is that he is a deceiver and he commands his worshipers to do bad things.
One example is happening now: He commands them to be in full fasting from dawn to sunset for a hole Lunar month (which just started) every year. They must not have anything enter their bodies, not even water, during daytime, regardless of how long it is or how hot it is. This can cause harmful dehydration.
"Do you want to guide those whom Allah has led astray? Whomever Allah leads astray—you will never find for him a way." 4:88
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