These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
From season 1's "522666", Raymond Dees is a Mad Bomber who derives sexual pleasure from the numerous deaths he causes. He initially blows up a crowded bar while watching the scene from afar. He later plants bombs in an office building, killing dozens more, and then participates in the rescue of the wounded simply so he can be hailed as a hero by the community. When Frank Black eventually discovers his true identity, Dees realizes that his game is up and arranges to be killed by the Feds so he can become notorious as a criminal instead, but not before taunting Frank with the possibility that he's rigged Frank's car to explode.
From season 1's "Maranatha", Yaponchik (aka Sergei Stepanovich, real name unknown) is a mass murderer who sees himself as an Antichrist figure whose mission it is to spread evil in the world. Taking inspiration from the Book of Revelation, he causes the Chernobyl disaster to kill many people and poison the land. A decade later in the United States, he kills a police officer and several people who tried to appease him, to re-establish his status as The Dreaded to the Russian community. He kills one of the two Russian officers who had been tracking him ever since Chernobyl by shooting him in public. He later goes on a killing spree in a bathhouse, before using religious fear to convince his nemesis Surova to help him escape so that he can continue his rampage elsewhere.
From season 2's "The Mikado", Avatar is a serial killer who terrorized the streets of San Francisco in the 1980s, claiming nineteen victims before eluding the authorities. He eventually returns to broadcast his latest murders over the internet, kidnapping several women and presenting them on a website with a counter indicating the time until their deaths. He leads the cops to a remote trailer, planting deadly boobytraps after killing the previous occupant. When Frank Black finds Avatar's new lair, he tries to kill Black as well before disguising his latest victim as himself in the hopes that Black will shoot her by mistake. Avatar never speaks, but his huge body count and his sadistic games make him one of the show's most heinous villains.
Broken Base: Many fans were understandably upset that the Fully Absorbed Finale featured Frank having a very small role while Mulder and Scully solved all his problems for him. Others say that seeing him get a happy ending is such a relief that they could overlook it.
Exactly, how did the Guardians determine who got to be a Chosen One? There seemed to be no distinguishing quality; one of them in fact turned out to be a racist bigot and was rejected. Another (Terra of the Teen Titans) was already dead, too. You'd think the Guardians would have known that.
What an Idiot: Harbinger, a character left over from Crisis on Infinite Earths, records the history of the universe in a device... which she leaves adrift in space for anyone to find! It was thanks to this that the Manhunters found out the secret identities of all the heroes.
Made worse by the fact that she barely gets reprimanded for it, and in fact joins the New Guardians.