Glowerworm
(aloof, craven, glower, & worm), someone who takes pride in or feels justified by scowling at someone else anonymously; expressing hostility towards someone, without verbally addressing that individual directly, privately or publicly; someone who conveys anger with another individual, without saying anything.
That guy scowling in the restaurant may be esteemed in his own community, but I now realize that he is either another glowerworm or just a figment of my twisted imagination.