Insanity: genuine or fake?

At certain points in the book, Grendel takes on an apathetic, more carefree attitude. This mainly takes form in Grendel acting nonsensical, and generally insane. Grendel's lack of regard for life, be it that of the animals, Thanes, or himself, it obvious from an early point in the book. However, one such jolly outburst comes out in the very beginning after real somber self-analysis, upon the edge of Grendel's favorite vast cliff. After a few depressing paragraphs of internal debate, he laughs it off all as some game he pretends to play with the cliff and skips off. Given that Grendel ends his life by throwing himself from the same cliff, he was obviously serious in his threats. Do you think that Grendel's insanity is real, or a coping mechanism to cover up for his depression and suicidal thoughts?