If Victor Zsasz is beige, Anton Arcane would be that awful green-brown made from mixing all the colours together.
Anton is scary for many of the same reasons the Tally Mark killer is, as his nightmarish human experimentation bares remarkable and unsettling resemblance to some dark parts of history - a concept which is not helped by Arcane's past with the Nazi party. While 'crazy unethical doctor' is a trope well used by comic villains, Anton also practises the occult, meaning between Satan and science its clear his hobbies are disturbing at best.
This is not to say its only Arcane's actions that make him terrifying as a villain, because he later has his brain implanted into one of his Un-Men, which has precisely one benefit - of having him look as awful and frightening on the outside, as he is on the inside.
It also added unnatural strength and a habit of being nude to his already nightmarish form, making him more closely resembling a Silent Hill monster than a villain from the same company who brought us Calendar Man.
All in all, if anyone can make Swamp Thing look normal in comparison, you know they're at least a little frightening.