Why do you think Grendel insists that his death is an accident?

Grendel insists that his death is an accident to downplay the role of fate in his demise and maintain the illusion of control over his life and actions. By framing it as an accident, he can deny responsibility for his actions and dismiss the idea of predetermined outcomes. It allows him to hold onto a sense of agency and choice in the face of his unavoidable fate.