In Beowulf, is Grendel an actual monster, a wicked man, or a symbol of immorality?

Grendel is indeed a very vicious monster, a fearsome, blood-thirsty creature waging a brutal war of terror and destruction against the Danes. What makes Grendel particularly scary is that there's no rhyme or reason to his wanton blood-lust; he appears to kill because it's in his nature to kill. And there's not much the Danes, or anyone else, can do to change that.
At the same time, there's something disturbingly human about Grendel's savagery. Then as now, there's sadly nothing unusual about the kind of bloodshed and suffering that Grendel inflicts with such gleeful abandon. Grendel's actions may be monstrous, but there's absolutely nothing he's done that hasn't already been done—and continues to be done—by so-called humans. In that sense, Grendel is a living embodiment of man's wickedness down the ages, with its callous disregard of the sanctity of human life.

I believe a reader could make a case for each of those labels about Grendel. I think it is legitimately possible to say that Grendel is all three as well.
Let us start with Grendel's morality. He is not a moral character. He is a murderous killing machine that eats people. That sounds pretty much like the antithesis of a moral character. In fact, it sounds eerily similar to the modern day character of Hannibal Lecter, and I do not believe anybody would say that he is a moral person. 
As for whether or not Grendel is a man or a nonhuman monster, I think he is human. I think that because of his ancestry:  

Grendel, who haunted the moors, the wild Marshes, and made his home in a hell Not hell but earth. He was spawned in that slime, Conceived by a pair of those monsters born Of Cain, murderous creatures banished By God, punished forever for the crime Of Abel's death.

Grendel is a descendant of Cain. In the Bible, Cain is guilty of fratricide, which is killing your own brother. Cain killed Abel, and God banished Cain. God also marked Cain and all of his future descendants. The story never says what the mark looks like, but it makes Cain clearly identifiable to anybody who sees him.
Because Grendel is a descendant of Cain (a human), I believe that Grendel is human. He is a demon-possessed, man-eating, violence-loving man, so I think it is safe to call him a monster as well.