Gregor plays the role of dutiful son to a domineering father until his transformation. The father has relinquished his role as family provider without giving up his sense of superiority. As a result, Gregor's transformation is first terrifying, and then perturbing. If Gregor did not work, who would pay off the father's debts and support the family?
Certainly Gregor's appearance is disturbing. The clerk who came to roust him to work tries to back out of the house quietly. The father, however, reacts with violence, using whatever weapon he could find (a walking stick and a rolled up newspaper) to force Gregor back into hiding. The father's response (including the "hissing") is much like the response of anyone who deals with an animal that is threatening a family member.
Gregor's father first viewed Gregor as simply the income earner, whose contribution was expected, not appreciated. After the metamorphosis Gregor's father viewed his son, not as a sone, but as an annoying animal that must be chased away no matter what.