Grendel's hatred of men is explained at the very beginning of the story. In the following passage, we find that he has been exiled from the happiness that is the Danish Kingdom:

So lived the clansmen in cheer and revel
a winsome life, till one began
to fashion evils, that field of hell.
Grendel this monster grim was called,
march-riever mighty, in moorland living,
in fen and fastness; fief of the giants
the hapless wight a while had kept
since the Creator his exile doomed.

Importantly, he is compared with Cain, who is one of the first people in the Bible to be exiled. Cain had killed his brother Abel, so there is reason for his exile. We don't know if Grendel had done something wrong, but this comparison is important. He may have done some evil deed to get himself exiled. Or, he may have just been hated by other men.

On kin of Cain was the killing avenged
by sovran God for slaughtered Abel.
Ill fared his feud, and far was he driven,
for the slaughter's sake, from sight of men.

The last part of the passage illustrates that there will be revenge. Cain was vengeful, and Grendel is vengeful:

Of Cain awoke all that woful breed,
Etins and elves and evil-spirits,
as well as the giants that warred with God
weary while: but their wage was paid them!

This is how the story begins. It sets up nicely to let the audience know why Grendel attacks Herot on a nightly basis.