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.
The words chosen will depend upon which translation of the text you are using. The second link below connects you with the enotes etext of Beowulf. Lines 56-59 of section II discuss the origin of Grendel. He is "The kindred of Caint" which points to his evil nature; Cain being the first murderer in the Bible. You can also consider the scene where Grendel first attacks. Lines 1-13 of section III call Grendel a "monster of evil greedy and cruel" and described how he killed thirty men and then went home "leaping and laughing" at his slaughter.