in Islam, Shirk is considered one of the greatest sins that a person can commit. Islam dictates that Shirk is never forgiven by Allah.
Shirk is defined as ascribing or the establishing "partners" placed beside Allah.
Muslims do not exactly realize this but they are committing technical Shirk by believing in Angels.
How is this Shirk? Angels in Islam are exactly identical to angels in Christianity and Judaism, meaning angels have immortality and have powers beyond that of humans.
By this definition, angels can be considered lesser gods. Since Muslims associate angels with Allah, they are thus committing an unforgivable sin.
Muslims do not generally think of angels as gods but angels definitely fit the definition of a god (i.e. being immortal and having powers beyond the power of humans)
thus, in conclusion, by believing in angels, Muslims are committing an unforgivable sin. From this, every Muslim that has ever believed in angels is in the fires of Hell.
I think this argument is clearly flawed. Just because X has an attribute of Y does not necessitate that X=Y. For example of this error in practice, apples exist and tables exist therefore, apples are at least some lesser form of tables.
Immortality is considered an attribute of God in all of the Abrahamic religions but it isn't necessarily an attribute that only God can have. In those religions, God can create beings that do not die without creating other gods. Eternal existence or, in this case, eternal preexistence would be the attribute in question since it is strictly reserved for God in all mainstream Abrahamic religions, and, as you would expect, no angels have this attribute in any of those religions.
Also, having supernatural powers does not necessitate deity. In the aforementioned religions, prophets and angels posses divine powers but they are not taken to be gods thereby. Most would simply state that these powers are authorised or endowed by God and even if that interpretation isn't held, it would be perfectly valid for someone to argue that they are not objects of worship and thus not deemed gods according to their religious model.
Islamic literature, like most religious literature, is rife with errors and contradictions but I'm afraid this is not from among them.