Even though Grendel seems as though he's being harsh by eating the exile, he is actually being practical. He says he tries to do things that are more considerate like befriending or ignoring him. However, since the exiles were treacherous, eating them is the most practical solution.
In this specific excerpt, Grendel is perfectly capable of logical reasoning as well as feelings. He does feel some sort of compassion for the exiles, which leads him to try and befriend them. But his intelligence also tells him to ignore them when it is convenient. In the end, his practicality wins over everything else. If the exiles cannot be trusted, if they are treacherous, it is best to get rid of them. If to get rid of them they are going to be killed, then it is best to eat them instead of letting their flesh go to waste.
Grendel was originally a character in the epic poem "Beowulf". He was portrayed as an evil monster that devours men. Author John Gardner, however, wrote a novel in which we get to see things from Grendel's perspective. In the novel, he is misunderstood and isolated by men due to his incapacity to express his thoughts and feelings.
