Internet Big Shot
Someone who, acting under the cover of internet anonymity, arrogantly and incessantly berates, argues with, or puts down others while masquerading as an expert on subject/s he or she knows nothing about.
John is such an internet big shot; despite his ignorance of medicine, he insists on internet message boards that Dr. Chu is an idiot for endorsing the utilization of palliative care measures for terminally ill patients.