Which words in the description of Grendel's birth are most effective in portraying his evil nature?

 In addition to the suggestions above, I think that the first reference to Grendel in line 30-31: The fire blasting demon/Ere long from from the hottest hatred must sword wrath.." is striking indication of his evil nature comparing it to the fires of hell.   Grendel could not bear the singing and music from the Hall.  He "bore it bitterly, he who bided in darkenss (line 34)" also is a stark portrayal of his evil.  Finally that he does his "deeds of direfullest malice (line 48)" I think is very effective in portraying his evil nature.  Each of these phrases creates a compelling picture of evil in the mind's eye.