4chan will cheer on almost any horrible thing you can come up with, but there are some things they will not countenance.
Child pornography, bomb threats, animal abuse (especially cat abuse) and anything related to criminal activity can and have led to them finding every bit of information about the poster they can find and turning it over to the police. Similarly, a Burger King employee in 2012 posted a picture of themselves standing in lettuce containers.◊ They traced him and others involved, and got them fired.
On /v/, one can occasionally find "rage threads" where /v/irgins attempt to piss each other off. While these threads may mostly consist of things like people on Facebook being stupid, Call of Duty fanboys, or My Little Pony fan art, you may run into something that could be considered morally wrong.
On a lighter note, pretty much any kind of sexual activity, real and fictional, will barely be blinked at, but consensual, normal, loving sex is jokingly treated as obscene.
/a/, the Anime and Manga board, has a code of honor regarding spoilers for Ever17. The same board that just loves dropping spoiler bombs on current hit shows actively discourages spoiling their favorite Visual Novel to newcomers.
Downplayed on the /r9k/ board in late 2014, when many of its users felt that Pepe the Frog, a comic character "adopted" by /r9k/, became too popular outside 4chan. In response, they drew pictures that depict him as a sadistic psychopath who does mostly scatological pranks. Some of the users found many of these pictures too disturbing and reacted by creating another meme that depicts Pepe as a helpful, nice person and a good friend to the other character who was his victim in the offensive "poo poo pee pee" images and comics.
/pol/, the "politically incorrect" board known for its racist and neo-Nazist content, also has "fat shaming" threads. In one of these threads in early 2015, an anonymous poster posted an image of a man who is first dancing and then sad because the poster and some others laughed at him. The rest of the board actually felt bad for the dancing man and were rude to the poster. The post then started a worldwide meme and a dancing party was held for the man in spring 2015.
When one of the boards of 4chan heard what happened to Steven Universe fan artist Zamii, they were disgusted at the people who caused her to attempt suicide. On a positive note, Zamii survived.
Deviant: Cerberus might be a group that has very little qualms about mowing down a bunch of gang mooks, but that's about the extent of any activities that could be considered "villainous", as they spend most of their time fighting people far worse than them. Notably, the Degenerates.
It is well-known for being flooded with fetishized pornography, real-life depictions of mutilation and torture, racist/sexist/homophobic commentary and mercilessly exploiting and demoralizing random people on the internet For the Lulz. But pedophiles and animal abusers surpass even furries on the list of people "EDiots" despise. If they're ever brought up in an ED article, expect them to be described as sick fucks, subhuman, the scum of the earth or some variation of the three.
Some of the rowdy admins on Facebook pages post stuff that mock and try to make events such as 9/11, the Dark Knight Shooting, suicide victims, deaths of popular people, Progeria, and such that are usually considered really offensive, but none of them have really tried to view the Sandy Hook Massacre in a brighter light. Although they aren't really popular or widespread.
One of the very few things that both sides of the powered community agree on is that family is not to be brought into the matter. For villains and heroes, their life being at risk is one thing — their kids being at risk just because of association? That's something else entirely, and people finding out about it is just about guaranteed to draw the ire of the whole community. And threatening the family of a Whateley student? The lucky ones are in jail for life. And on life support — also for life. When a Religion of Evil named 'The Tong of The Black Madonna' tried to pressure Bladedancer by leaning on the family of her girlfriend Gateway, the Whateley Alumni Association in conjunction with the Syndicate basically declared open season on everyone and everything associated with them - anyone who contributed to destroying them would be given assistance, and benefits (such getting any booty they take from them, or being excused of past debts to The Syndicate), for doing so. One of the parents, the Affably EvilWell-Intentioned Extremist Dr. Diabolik, suggested that Orbital Bombardment might constitute a 'measured response' to the Tong's actions. This was considered a relatively mild suggestion by most of the other supervillains (and probably some of the superheroes as well).
Mephisto: Now, I'll admit that the Brotherhood doesn't play nice. Hell, by the standards that the vast majority of the populace subscribes to, we're evil as hell. But, we have the long-term good of the human race at heart. Selling kids to the Pit does NOT serve the greater good in any way, shape or form. There is shit even we don't put up with...
Dr. Venus: The rest of us do horrible, illegal things... but YOU? You experiment on KIDS! [...] You grab kids off the street and shove them literally kicking and screaming into that Monster Chamber of yours! You turn them into fucking monsters! That's sick, Cobb, SICK!
The supervillains of Brockton Bay include the Neo-Nazi group Empire 88, the Diabolical Mastermind Coil, the mercenaries of Faultline's Crew, and the Undersiders. However, they all look down on Skidmark and his gang, who are incompetent drug-dealers who sell to kids. More in line with the spirit of the trope, they are also all willing to join forces against psychopaths like the Slaughterhouse Nine.
Bonesaw, a member of the Slaughterhouse Nine, gleefully delights in inflicting Body Horror of the worst kind, but in her own words, even she isn't crazy enough to try to clone an Endbringer.
Before the story starts a notorious and extremely powerful villain named Marquis refuses to attack women and children, the heroes use this fact and the fact that he is trying to protect his daughter Panacea to capture him.