I think people who say that really mean, “Any God who would create/allow Hell is not good but evil, and you say God is good.”
The Muslims believe that good is what Allah says it is - and that he could say anything. If he decides to say giving alms is evil and stealing is good, it is so.
Suppose the Muslim concept of God and goodness were true, not the Christian one. I suppose most of us would become Muslims to avoid hell, if we knew that was the only way - but wouldn’t the heart you have now cry out against the teachings of Islam, slavery and conquest and murder, cry out that it was still evil?
The Muslims certainly do not reject Allah because of Jahannam (the Koran’s “Hell”).
Evil is what God says, but He says it for a reason, unlike the arbitrary Allah