I think the feelings of Gregor Samsa the first sentence gives you is not accurately translated in the English version. The ungeheures (uncanny/giant/unheard of) Ungeziefer (unpleasant and unwanted insects like cockroaches are Ungeziefer, insects people/society don't like) is a metaphor for how he feels/is treated by his family; not really a physical state. Based on how Samsa then treats this metamorphosis as a practical problem to overcome and live his "normal" life, I always felt like this physical state is simply a manifestation of his/Kafka's inability to follow the rules of and take part in society just by being too different. The rules of society (and his father/family) become impossible to follow, and the uncannyness of one's existence expose the uncanny rules of the middle classes Gregor Samsa desperately wants to participate in. But as the rules define the life of everyone in this world and don't allow one to be different, it destroys him in the end. Being uncanny and unwanted by society dooms the protagonist from the first moment (metaphorically he is doomed to perish by being hurt with the apple by his beloved sister in the beginning).