Throughout the novel, Grendel talks about his own view of life and what it means, and although everyones views of life are different, Grendels was far different from the majority of the world at the time. Grendels own view of life is that there is no real "purpose" of life, which separated him from everyone else. He never tried to find his purpose in life, so all he ever did was wander the world and observe life, and the way humans and animals interacted with eachother.
In a way, Grendel made himself an outsider. He would not allow himself to be associated with humans, so his views are almost polar opposites of theirs, which in turn, made him an "outsider". Not only did Gendels views of life make him an outsider, but the relationship he had with humans was a major part of him being separated from the majority of society. The humans thought that Grendel was a monster, so when he would come to the villages, they would all start attacking him, and attempt to drive him out.
This would obviously make him an outsider, because none of the humans wanted to be around him, so he was forced to be on his own. However, after a while, Grendel began to hate the humans. He would go into Herot and kill people because he hated them so much. This widened the social gap between the humans and Gredel, making him even more so of an outsider. Grendels relationship with his mother is also a major factor in Grendel separation from society.
While Grendel is an intelligent being, who is capable of speech, his mother has never been able to speak. This is a key factor in the development of Grendel. He was never taught the way "others" would have been taught. Instead of learning speach from his mother, he learned from hearing the humans talk, and slowly began to pick up on the language. He taught himself how to do most of the things that he does now, which makes him "stand out from the crowd".
Also, it is hard for him an his mother to communicate well with eachother, which could potentially make Grendel even more of an outsider, because of the lack of communication between him and his mother. Grendels views and relationships truly separate him from the rest of mainstream society. His strange and alien morals and ideals make him an outsider among the humans, and the rest of the world that he lives in. He truly is the only one of his kind.